Guys,
perhaps I got the solution. I think wing incidence is the motivation. Let me explain why.

Let's imagine a wing with a great AoA needed to fly straight level (great so that we can visualize it into our mind). This could be the case of a nose heavy airplane. Well, in such a configuration, suppose we apply full ailerons in one direction. Now, the aileron that moves downward "sees" a laminar air flow, whereas the aileron that moves upward "sees" a little turbolent air flow (the more incidence, the more turbolent as air flow will leave wing surface at increasing AoA).
So, if I am correct up to now, the solution is simple: the downward generate more lift and hence more drag, so its throw must be reduced. So far, so good.

Now let's imagine an inverted flight condition. The theory remains the same: even in this situation, the upward aileron (which previously was moving downward) sees a laminar flow so its throw must be reduced.

If these theory is correct, we can say that greater wing incidence will make the need for greater differetials.

A little check can be done now: Angus, you told us that in your Synergy you have 22% diff. and only 7% in your Alliance. What can you say about wing incidence of your planes? Is perhaps greater the one of Synergy (cosidering of course also elevator trim)?

Also, I think that wing airfoil must be considered too. Perhaps an airfoil produces more turbolence at low AoA (tipically required to do a level flight, i.e. 0.5-1 degree) than other, so the comparison between two different planes is not significant...

What do you think?