Hmmmm, that's what I thought. Although when I ran it with my centered variable it came out identical to my original analysis without a covariate, which is a little unrealistic.

I centered Age within each group and then put the centered age in my syntax (below). Am I missing something?

3dMVM -prefix ANOVA_rECN_ageCOV_GRPcenter \
-mask MNI152_T1_2mm_brain_mask.nii.gz \
-bsVars Grp \
-qVars Age \
-qVarCenters 0 \
-dataTable \
Subj Grp Age InputFile \
s1 HCC -0.71536 dr_stage2_subject00000_Z.nii.gz'[1]' \
s2 HCC 1.06732 dr_stage2_subject00001_Z.nii.gz'[1]' \
s3 HCC -0.71536 dr_stage2_subject00002_Z.nii.gz'[1]' \
s4 HCC -0.18055 dr_stage2_subject00003_Z.nii.gz'[1]' \
s5 HCC -1.07189 dr_stage2_subject00004_Z.nii.gz'[1]' \
s6 HCC -0.18055 dr_stage2_subject00005_Z.nii.gz'[1]' \
s7 HCC -0.80449 dr_stage2_subject00006_Z.nii.gz'[1]' \
s8 HCC -0.71536 dr_stage2_subject00007_Z.nii.gz'[1]' \
s9 HCC 1.42386 dr_stage2_subject00008_Z.nii.gz'[1]' \
s10 HCC -0.71536 dr_stage2_subject00009_Z.nii.gz'[1]' \
s11 HCC -0.62622 dr_stage2_subject00010_Z.nii.gz'[1]' \
s12 HCC -0.35882 dr_stage2_subject00011_Z.nii.gz'[1]' \
s13 HCC -0.44795 dr_stage2_subject00012_Z.nii.gz'[1]' \
s14 HCC -1.07189 dr_stage2_subject00056_Z.nii.gz'[1]' \
s15 HCC -0.80449 dr_stage2_subject00013_Z.nii.gz'[1]' \
s16 HCC -0.71536 dr_stage2_subject00014_Z.nii.gz'[1]' \
s17 HCC -0.35882 dr_stage2_subject00015_Z.nii.gz'[1]' \
s18 HCC -0.62622 dr_stage2_subject00016_Z.nii.gz'[1]' \
s19 HCC -0.98276 dr_stage2_subject00017_Z.nii.gz'[1]' \
s20 HCC -0.18055 dr_stage2_subject00018_Z.nii.gz'[1]' \
s21 HCC 0.35425 dr_stage2_subject00019_Z.nii.gz'[1]' \
s22 HCC -0.53709 dr_stage2_subject00020_Z.nii.gz'[1]' \
s23 HCC 1.86953 dr_stage2_subject00021_Z.nii.gz'[1]' \
s24 HCC -0.18055 dr_stage2_subject00022_Z.nii.gz'[1]' \
s25 HCC 2.49346 dr_stage2_subject00023_Z.nii.gz'[1]' \
s26 HCC 2.40433 dr_stage2_subject00024_Z.nii.gz'[1]' \
s27 HCC 0.44338 dr_stage2_subject00025_Z.nii.gz'[1]' \
s28 HCC 1.51299 dr_stage2_subject00026_Z.nii.gz'[1]' \
s29 HCC 1.51299 dr_stage2_subject00027_Z.nii.gz'[1]' \
s30 HCC 0.62165 dr_stage2_subject00028_Z.nii.gz'[1]' \
s31 HCC 0.53252 dr_stage2_subject00029_Z.nii.gz'[1]' \
s32 HCC -0.53709 dr_stage2_subject00030_Z.nii.gz'[1]' \
s33 HCC -0.35882 dr_stage2_subject00031_Z.nii.gz'[1]' \
s34 HCC 1.60212 dr_stage2_subject00032_Z.nii.gz'[1]' \
s35 HCC -0.80449 dr_stage2_subject00033_Z.nii.gz'[1]' \
s36 HCC -0.89362 dr_stage2_subject00034_Z.nii.gz'[1]' \
s37 HCC -0.44795 dr_stage2_subject00035_Z.nii.gz'[1]' \
s38 HCC -0.26969 dr_stage2_subject00036_Z.nii.gz'[1]' \
s39 HCC -0.53709 dr_stage2_subject00037_Z.nii.gz'[1]' \
s40 LCC 1.24464 dr_stage2_subject00038_Z.nii.gz'[1]' \
s41 LCC -1.44737 dr_stage2_subject00039_Z.nii.gz'[1]' \
s42 LCC -1.17817 dr_stage2_subject00040_Z.nii.gz'[1]' \
s43 LCC -0.63977 dr_stage2_subject00041_Z.nii.gz'[1]' \
s44 LCC -0.9987 dr_stage2_subject00042_Z.nii.gz'[1]' \
s45 LCC 1.60358 dr_stage2_subject00043_Z.nii.gz'[1]' \
s46 LCC -0.37057 dr_stage2_subject00044_Z.nii.gz'[1]' \
s47 LCC -1.53711 dr_stage2_subject00045_Z.nii.gz'[1]' \
s48 LCC -1.35764 dr_stage2_subject00046_Z.nii.gz'[1]' \
s49 LCC -0.4603 dr_stage2_subject00047_Z.nii.gz'[1]' \
s50 LCC -0.01163 dr_stage2_subject00048_Z.nii.gz'[1]' \
s51 LCC -0.9987 dr_stage2_subject00049_Z.nii.gz'[1]' \
s52 LCC -0.37057 dr_stage2_subject00050_Z.nii.gz'[1]' \
s53 LCC -1.08844 dr_stage2_subject00051_Z.nii.gz'[1]' \
s54 LCC -0.63977 dr_stage2_subject00052_Z.nii.gz'[1]' \
s55 LCC -1.26791 dr_stage2_subject00053_Z.nii.gz'[1]' \
s56 LCC -0.28083 dr_stage2_subject00054_Z.nii.gz'[1]' \
s57 LCC -0.90897 dr_stage2_subject00055_Z.nii.gz'[1]' \
s58 LCC 1.06517 dr_stage2_subject00057_Z.nii.gz'[1]' \
s59 LCC 0.79597 dr_stage2_subject00058_Z.nii.gz'[1]' \
s60 LCC -0.4603 dr_stage2_subject00059_Z.nii.gz'[1]' \
s61 LCC -0.37057 dr_stage2_subject00060_Z.nii.gz'[1]' \
s62 LCC 0.79597 dr_stage2_subject00061_Z.nii.gz'[1]' \
s63 LCC -0.37057 dr_stage2_subject00062_Z.nii.gz'[1]' \
s64 LCC 1.33437 dr_stage2_subject00063_Z.nii.gz'[1]' \
s65 LCC -0.81924 dr_stage2_subject00064_Z.nii.gz'[1]' \
s66 LCC 1.24464 dr_stage2_subject00065_Z.nii.gz'[1]' \
s67 LCC 0.6165 dr_stage2_subject00066_Z.nii.gz'[1]' \
s68 LCC 1.78304 dr_stage2_subject00067_Z.nii.gz'[1]' \
s69 LCC -0.7295 dr_stage2_subject00068_Z.nii.gz'[1]' \
s70 LCC -0.4603 dr_stage2_subject00069_Z.nii.gz'[1]' \
s71 LCC -0.1911 dr_stage2_subject00070_Z.nii.gz'[1]' \
s72 LCC 2.50091 dr_stage2_subject00071_Z.nii.gz'[1]' \
s73 LCC 0.43704 dr_stage2_subject00072_Z.nii.gz'[1]' \
s74 LCC -0.4603 dr_stage2_subject00073_Z.nii.gz'[1]' \
s75 LCC -0.4603 dr_stage2_subject00074_Z.nii.gz'[1]' \
s76 LCC -0.10137 dr_stage2_subject00075_Z.nii.gz'[1]' \
s77 LCC 0.6165 dr_stage2_subject00076_Z.nii.gz'[1]' \
s78 LCC 0.88571 dr_stage2_subject00077_Z.nii.gz'[1]' \
s79 LCC 0.79597 dr_stage2_subject00078_Z.nii.gz'[1]' \
s80 LCC 2.14198 dr_stage2_subject00079_Z.nii.gz'[1]' \
s81 LCC -0.4603 dr_stage2_subject00080_Z.nii.gz'[1]' \
s82 LCC 1.78304 dr_stage2_subject00081_Z.nii.gz'[1]' \
s83 LCC 0.79597 dr_stage2_subject00082_Z.nii.gz'[1]' \
s84 LCC -0.28083 dr_stage2_subject00083_Z.nii.gz'[1]' \
s85 LCC 0.43704 dr_stage2_subject00084_Z.nii.gz'[1]' \
s86 LCC -0.10137 dr_stage2_subject00085_Z.nii.gz'[1]' \
s87 LCC 0.6165 dr_stage2_subject00086_Z.nii.gz'[1]' \
s88 LCC -0.7295 dr_stage2_subject00087_Z.nii.gz'[1]' \
s89 LCC -1.35764 dr_stage2_subject00088_Z.nii.gz'[1]' \
s90 LCC -0.81924 dr_stage2_subject00089_Z.nii.gz'[1]' \
s91 LCC 0.0781 dr_stage2_subject00090_Z.nii.gz'[1]' \
s92 LCC 1.06517 dr_stage2_subject00091_Z.nii.gz'[1]' \
s93 LCC -0.90897 dr_stage2_subject00092_Z.nii.gz'[1]' \
s94 SZ 0.94194 dr_stage2_subject00093_Z.nii.gz'[1]' \
s95 SZ -1.6737 dr_stage2_subject00094_Z.nii.gz'[1]' \
s96 SZ -0.27246 dr_stage2_subject00095_Z.nii.gz'[1]' \
s97 SZ -0.27246 dr_stage2_subject00096_Z.nii.gz'[1]' \
s98 SZ 0.94194 dr_stage2_subject00097_Z.nii.gz'[1]' \
s99 SZ 1.40902 dr_stage2_subject00098_Z.nii.gz'[1]' \
s100 SZ 0.56828 dr_stage2_subject00099_Z.nii.gz'[1]' \
s101 SZ 0.94194 dr_stage2_subject00100_Z.nii.gz'[1]' \
s102 SZ 0.56828 dr_stage2_subject00101_Z.nii.gz'[1]' \
s103 SZ 1.12878 dr_stage2_subject00102_Z.nii.gz'[1]' \
s104 SZ -1.39346 dr_stage2_subject00103_Z.nii.gz'[1]' \
s105 SZ 1.22219 dr_stage2_subject00104_Z.nii.gz'[1]' \
s106 SZ 0.00778 dr_stage2_subject00105_Z.nii.gz'[1]' \
s107 SZ -0.08563 dr_stage2_subject00106_Z.nii.gz'[1]' \
s108 SZ 1.03536 dr_stage2_subject00107_Z.nii.gz'[1]' \
s109 SZ -1.76712 dr_stage2_subject00108_Z.nii.gz'[1]' \
s110 SZ -1.86054 dr_stage2_subject00109_Z.nii.gz'[1]' \
s111 SZ 1.22219 dr_stage2_subject00110_Z.nii.gz'[1]' \
s112 SZ -0.83296 dr_stage2_subject00111_Z.nii.gz'[1]' \
s113 SZ 0.47486 dr_stage2_subject00112_Z.nii.gz'[1]' \
s114 SZ 0.1012 dr_stage2_subject00113_Z.nii.gz'[1]' \
s115 SZ -1.30004 dr_stage2_subject00114_Z.nii.gz'[1]' \
s116 SZ 0.56828 dr_stage2_subject00115_Z.nii.gz'[1]' \
s117 SZ 0.56828 dr_stage2_subject00116_Z.nii.gz'[1]' \
s118 SZ 0.1012 dr_stage2_subject00117_Z.nii.gz'[1]' \
s119 SZ 1.12878 dr_stage2_subject00118_Z.nii.gz'[1]' \
s120 SZ -1.11321 dr_stage2_subject00119_Z.nii.gz'[1]' \
s121 SZ -1.11321 dr_stage2_subject00120_Z.nii.gz'[1]' \
s122 SZ -0.55271 dr_stage2_subject00121_Z.nii.gz'[1]' \
s123 SZ -1.30004 dr_stage2_subject00122_Z.nii.gz'[1]' \
s124 SZ 0.75511 dr_stage2_subject00123_Z.nii.gz'[1]' \
s125 SZ 0.19462 dr_stage2_subject00124_Z.nii.gz'[1]' \
s126 SZ 1.12878 dr_stage2_subject00125_Z.nii.gz'[1]' \
s127 SZ 0.28803 dr_stage2_subject00126_Z.nii.gz'[1]' \
s128 SZ -1.30004 dr_stage2_subject00127_Z.nii.gz'[1]' \
s129 SZ -0.4593 dr_stage2_subject00128_Z.nii.gz'[1]'