[Alan Anderson]

Many motors have a measurable bias in their "forward" direction. However, a CIM's bias is negligible.

You can't compensate for bias by swapping wires. There is no electrical difference between sending full positive voltage to a motor's black wire and sending full negative voltage to its red wire. Assuming the same mechanical load, it will spin in the same direction at the same speed in either case.

The place to look is in the mechanical side of things, not electrical. The slower-moving side could have some extra friction or binding, perhaps due to a too-tight chain or belt. Or it might just be that the wheels aren't mounted quite in line with the desired robot travel and the chassis would tend to travel in a curve even when being pushed by hand with no power applied.