I had a similar issue: among 1441 cases, in 6 occasions 3dcalc changed the number of voxels in the x dimension from the original value to the original value + 1.
I regularly work with nifti files.
I understand that I can copy the header from the original file to the output of 3dcalc to bypass this issue, but I would like to avoid doing that, as the output of 3dcalc is supposed to always have the same dimension of the inputs.

Is there a way to solve this issue in 3dcalc? In theory, the number of voxels in any dimension should always be an integer number, so I do not understand why 3dcalc would have a "rounding problem".

Thank you very much.

Giuseppe