[Jared Russell]

Possible explanations:

1. With manufacturing tolerances and different wear patterns, it is possible that the right CIMs are "slower but torquier" and the left CIMs are "faster but less torquey". There could also be mismatch and therefore non-equal current draw within an individual gearbox, if there are more than one motor per gearbox.

2. Different retarding forces on each side, including both "coulomb" type stick-slip friction (~proportional to acceleration) and viscous damping (~proportional to speed).

3. Different lengths of wire or quality of crimps between the sides resulting in more or less current reaching the actual motors.

4. Unequal mass.

5. Damaged CIMs. A toasted motor continues to run, but draws more current and produces less mechanical power.

I don't know how much time you want to sink into this project, but a some relatively simple experiments you could run would be swapping CIMs around, measuring wire/crimp resistance with a multimeter, bench testing each CIM/gearbox, swapping whole transmissions left/right, and measuring and balancing weight on each set of wheels.