You can only get the discounted rate if there is a room available at that resort.
Most people only book what they can afford, in case there aren't discount rates.
I'm copying this from susy post's in another thread because I think this explains it well:
The more rooms that are booked ahead of time, the less the rooms that will be discounted. Codes are not "applied" to your room. Your room is sold when you book it (at rack rate). When you get a discount, it's on a room that has not yet been sold. So, if there are no rooms available at the code rate, you pay the rate you agreed to