Dealers can sometimes set the rate earlier, however they need to agree this with the finance company first.
I paid a deposit last week, my dealer would only guarantee a finance rate if I signed a finance contract then, if I did it at delivery I would only have what the current (in June, when the car is delivered) deal was\is. The only reason I didn't sign is that I will probably not use any dealer finance.
I hope your deal is really good, I've heard that 10-12% is easy to achive at the moment, even with such a long waiting list.