Thanks for spotting this, its not something I'd noticed before. Here's what's happening:
Reaper is choosing which frame to show by rounding down. When a stack has a low frame count like this (I think 35 frames is low but adequate for a small pan knob) then there's an offset throughout because the first frame covers all the way from 0 to 95%L, but the last frame is only used for
exactly and
only exactly 100%R.
The default theme uses the code pointers, not bitmap stacks, because they may not look very good but they are flawlessly accurate at any size.