That's one of the many tactics car dealerships use. If you want the model with some fancy options then you can drive it away today. If you want a car without additional extras then it's not in stock, so you'll have to wait 4 weeks (or more) for delivery, but the finance deal they're offering has to be taken before delivery of the car and it expires in a couple of weeks. Consequently you can only get finance if you pay for the options...