I always thought it had to do with the fact that lift coefficients were changed when the aileron deflected, meaning the downward moving aileron INCREASED lift, and the upward moving increased DRAG, thus causing the yaw. ALthough most pattern airplanes these day have the wing location on the fuse correct to offset this, years ago, it was a real problem (lower wing placement on fuse increases need for differential)

BUT I could be wrong, here....