The Toe in may be the culprit. I have the same thing going on with ONE front tire(left front). Car tracks like it should no pull upon braking and no pulling to one side of the road or the other. Yet, the left front tire is worn drastically on the outside edge. All other tires are fine. I thought Caster or camber may be off on just this wheel, but now i will have the alignment checked and not only the caster and camber, but the toe in.
Toed inward too much would cause that, in my way of thinking.
I have only owned the 560 SEC for a month and it has great tires on it(well at least 3 great ones and one scarfing the Bejeebers out of the outside edge...LOL!)