You got to remember, a dealer makes very little on a Motability sale, about £500. They make many times that on a private sale.
Its not that Motability are lesser customers, its just the dealerships have a percentage for retail sales and a low percentage for Motability sales. Its nothing personal, its just maximising profitability.
However, if a sales are slow or the dealers have excess stock of older models, then they discount them to Motability to generate sales.