Let's see if an example would help:
Checkin / Checkout / 11 month window
June 29, 2003 / June 30, 2003 / July 30, 2002
June 30, 2003 / July 1, 2003 / August 1, 2002
July 1, 2003 / July 2, 2003 / August 2, 2002
July 2, 2003 / July 3, 2003 / August 5, 2002 (MS closed on weekend)
July 3, 2003 / July 4, 2003 / August 5, 2002 (ditto)
July 4, 2003 / July 5, 2003 / August 5, 2002
So there you have it. And example of what Doctor P was talking about would be:
Checkin / Checkout / 11 month window
August 29, 2003 / August 30, 2003 / September 30, 2002
August 30, 2003 / August 31, 2003 / September 30, 2002
August 31, 2003 / September 1, 2003 / October 1, 2002
Of course this is a horrible example because MS is closed on 9/30 and 10/1 this year, but you get the idea. The craziest month to reserve using the 11 month window is January because February is so short. You can reserve a rooms with a checkout date of January 28, 29, 30, and 31st on February 28th. That is unless there is a February 29th that year. If you think this is confusing, think of the poor programmer who had to program the reservation system to handle this rule.
I have no clue as to why you were able to book the room today. Yet another example of Disney Magic I guess.