Howdy-
How did you convert the mask to subject space? With alignment of some kind?
To check if the grid is really the same and get a list of detailed comparisons, you can run this, putting in your statistical and mask dsets, respectively:
3dinfo -same_all_grid -prefix DSET_STAT DSET_MASK
It might also be useful to know:
3dinfo -space -av_space -prefix DSET_STAT DSET_MASK
--pt