So we rig the ailerons to move with differential.

And we have two ways that differential can help that airplane. We can stop the downgoing aileron from creating more drag so fast and so much.

The other side isn't so simple. We need to get that aileron producing drag, and to do it quickly. With most airfoils that wind up needing differential ailerons on them, move the aileron up just a little and you won't get much differential effect. And the other wing/aileron has had it's throw reduced, so you've got an airplane that has less effective ailerons right around zero deflection.

So you want differential that move the up aileron up faster and considerably more than before. Well, depending on the wing's airfoil.

Nope, aerodynamics ain't sound byte simple.