To deepen the discussion a bit...

Why do some 'neutral' aerobatic planes (0 wing incidence, 0 tail incidence, center-hinged surfaces, etc.) also exhibit the need for differential? When fine tuning an aircraft for competition, you can climb high and establish a perfectly vertical power-off downline, and then roll continuously. Many planes, even with exactly equal throws measured up and down, you can see a slight barrel to the roll, which differential fixes. Obviously, a vertical line is used because in a horizontal roll you must input rudder to keep it axial, so it's hard to base setup changes on that, and a vertical downline is chosen to reduce any impact thrustline may have on how axial the roll is. It is a very small amount - almost imperceptible - but it's noticable sometimes if you're looking to really fine tune your setup.

P.S. The only time I've seen a relatively conventional plane require more down aileron than up is in the case of a top skin hinge, where the up deflection opens up a large gap in the wing creating drag and the down aileron closes the gap resulting in less drag. (i.e. FiberClassics/Composite-ARF... the screeching noise