Hi Ryan,
I had never noticed this because I don't have compression set on my machine. 3drefit is doing the origin shift (from the @Align_Centers command) and because - I suspect - you have AFNI_COMPRESSOR set to GZIP it is writing out a .nii.gz file even if the input is a .nii.
My impulse is to keep 3drefit from changing the compression in the first place, but I have to consider this carefully first. In the meanwhile, you can either set AFNI_COMPRESSOR to NONE before you run the script, or just delete the .nii files if you have .nii.gz too.
I should have this resolved one way or the other by the next build.
cheers,
z