I suspect the problem is that Motability do not know what cars are going to be available or at what price so far ahead. In the past we could depend on cars offered each quarter being available throughout the quarter and the AP was fixed as long as we ordered in that quarter, regardless of later price changes. This is no longer the case as cars are removed and added seemingly at random.
Unfortunately there is no stability in the new car market regards availability or price. We are in difficult times…