Another reason is that the trust line of the engine is normally not at the center of the wing. As is the center of gravity, this is also normally not at the center of the wing. The plane however will roll around a line combined by the the CG and the trust line. The further the wings is away from this line, the more differential is needed to roll axial.

Compare a real low-wing plane (CAP) or a high wing (Decathlon) plane with a symatrical wing. The need a reverse differntial to roll axial. The low wing plane needs more down then up aileron, the highwing needs more up then down.

At least, that is my understanding.