Hi,
with n slices there are n+1 positions (slice boundaries) where playback can start and stop. So if the knob had a range of 0 to 127 you would be able to select 128 positions and that would give you 127 slices. You would not be able to select any perfect divisions because 127 is a prime. To get 128 slices the knob would need to range from 0 to 128 (have 129 values) and thus break the MIDI 7-bit convention which allows only 128 values for a parameter.
With a range of 0 to 120 you have 121 positions available. The number of slices is 120 and this number divides with 2, 3, 4, 5, and 8. So you can play for instance one beat from a 3/4 or 5/4 bar, or one eight from a 4/4 bar.
I hope this makes sense 