It could also be the fit of the hob to the spindle. If I recall correctly before the advent of hydraulic spindles gear hobs had different tolerance fits to the spindle much the same as thread classes. I do recall some bobs being identified with a fit of H5 and H6. The tighter the tolerance the lower the number. This was more important for precision gears when all components of the machine were mechanical. The hob had to fit precisely to insure proper depth and clearance. Now that most machines have hydraulically controlled spindles they can keep uniform and constant pressure against the work piece so an .0005" difference in fit to the spindle can be compensated for.