It just means that there are no discounted rooms left. They only allocate a small percentage to the AP discount. There is also the Military Discount program in effect until June 23, plus any conventions/conferences that have rooms blocked, weddings, etc. The AP discount rooms are not ALL the rooms available at the hotel at any given time. I think they allocates something like 10% of the ENTIRE inventory to the AP rate, so just something to think about.
There could also be something going on...Grad Nights are a possibility, but they don't book rooms for that.
I do know that they recently extended the AP8 promotion to mid June, so maybe that actually worked, and people booked up the rooms!