I would imagine that the manufacturer negotiated with Motobility to determine a purchase price and therefore the AP. As most dealers are independent franchises who buy their cars from the manufacturer at a discount which is dependant on volume and other incentives, each manufacturer would have its own payment schedule for Motobility cars sold. It may well change with occasional promotions on certain vehicles
This all means that different dealerships pay their sales Ent different amounts depending on the incentives.
When I got my Mazda, the salesman told me that the dealership got £500 per CX5 but it didn’t count towards their unit volume.