Pan has 128 positions for all devices using the midi protocol, it's standard. Now if Apple were to change this in any way, other devices communicating via midi to Logic would get confused.
The pan position 63 1/2 does not exist in Midi, so a compromise has to be reached.
Apple haven't changed this because they assume we're all professionals, know this fact and would complain if Logic didn't follow the Midi standard.
The reason it's 128, and not 130 or 150 is because midi is a binary protocol based on an 8 bit system 8, 16, 32, 64, 128, 256 etc. etc.
That's why you have 16 midi channels and 128 positions of volume, or your synth may have 8 banks of 32 sound or 16 banks of 64 sounds or any other combination of 8's
It's got nothing to do with Apple. Period.