CIM motors are brushed motors. Brushed motors have the tendency for this to occur (thread regarding phenomenon:
http://forums.overclockers.com.au/sh...php?t=1063875; sorry I couldn't find better source, I'm tired/long day at work).
You can actually hear the motor sound differently free-spinning in opposite directions if you directly connect it to a battery. One direction has a higher pitch whine than the other.
Admittedly, most motors I've used are a couple years old, so that may (?) have something to do with why my observed difference is so high (brushes are no longer perfectly distributed?), but yeah, common occurrence. Even last year when we built w/ KOP chassis, had to account for it in code for auto programming. Our 2014 robot had a pretty bad arc before our programmer added the correction.
Also, I'm generally more mechanical in thinking, so hopefully someone more electronically intelligent can fill in the gaps to my explanation.