robert & basmntdweller , you're arriving at the same conclusion in a different way.

Here is a mental experiment:
Suppose I set up my plane to fly inverted most of the time. It has a symmetrical airfoil and must fly at a slightly positive angle of attack, relative to earth ground, to keep the airplane aloft. I set my aileron differential to eliminate all adverse yaw.

Now, when I flip my plane upright, what do I find? The up aileron is 20 degrees high while the down aileron is 30 degrees low. It's exactly backward from what it should be.

So what changed? The small angle of attack required to keep the plane in the air is equal relative to earth ground but opposite relative to the airplane upright compared to inverted. It's the positive angle of attack, or lift coefficient, that causes adverse yaw and ONE of the purposes for differential aileron control.

The other purpose is wing position, dihedral, and other airframe geometry.

That's the way I learned it anyhow.