Dear Paul,
seems that your suggestion to NOT include the option "-in_ref_orig .." solve my problem, now all subject Geometry string outputs are in "exactly" expected form:
Geometry String: "MATRIX(1.5,0,0,-94.75,0,1.5,0,-99.75,0,0,1.5,-72.75):128,157,115"
Geometry String: "MATRIX(1.5,0,0,-94.75,0,1.5,0,-99.75,0,0,1.5,-72.75):128,157,115"
Geometry String: "MATRIX(1.5,0,0,-94.75,0,1.5,0,-99.75,0,0,1.5,-72.75):128,157,115"
Geometry String: "MATRIX(1.5,0,0,-94.75,0,1.5,0,-99.75,0,0,1.5,-72.75):128,157,115"

I also generated the -mask from the structural output of Tortoise, which as the same Geometry string and the same voxel size (1.5x1.5x1.5), while the imit2w_orig_ss has generally different geometry.

Thank you very much for your valuable help,
cheers,
Giuseppe