I am not a leasing expert, but that sort of makes sense to me. All cars have some mileage on them when leased, and you don't credit for them if you go over the allotted amount. However, the dealer should be able to make up for that by lowering the price on which the lease is based upon.
I'm not an expert by any means either, but I know my way through a lease. Leasing a demo is not something I'm terribly well versed with. In my case, what the dealer was discounting the Genesis by was not enough for it to make sense.