[Mithrandir]

I think this might explain...

A model is stable. Usually this means that upright it will fly straight and level. Roll'er inverted and she will slowly dive towards the ground.
This stability is generally achieved by having the wing at some positive angle of attack with respect to the stabilizer. What differential does, is counteract this relative angle during the aileron input, so that the "Spiral" tendency is reduced.
The wing with the UP aileron has had its angle of attack with respect to the stab reduced more then the other side was increased because it started out with a positive angle initially.

(I think this is what prompts differential aileron)

Think about this... in a rolling Harrier, the wing is always lifting "UP"..... so if rolling left, the plane naturally wants to circle right.... in this instance, the wing that has the "Down" deflected aileron is dragging the plane to the right... (Down with respect to the earth and NOT the plane)... so upright or inverted, there is drag on the right side....
I bet if we mixed differential ailerons to the elevator, so that as the elevator was applied in the rolling harrier, the deflection of the down aileron was reduced, this tendency would be diminished... basically a dynamic differential mixed to elevator....