Because the system rechecks availability when you click book. Which you select the 11 month date as your first night. However, when you recheck availability and the 1st night is no longer open the system moves to the largest available block of rooms then tries to process the request. Since that largest available block of rooms no longer includes the 11 month date, it is now trying to book a block of rooms outside your booking window. It is a bad error message and handling on their part and they shouldn't be readjusting your selected dates by clicking book but they are.
I assume you are going for a CCV Studio, AKV Value/Club, BLT Standard, BWV Standard/Boardwalk, RIV Tower, or RIV Standard. My guess is all those rooms are being walked and have been walked for the past couple weeks to make sure people were getting the marathon weekend.