Disney probably wants to keep "blocks" of days available for that room, because they know they can rent the room in a 3 night and 5 night block....or a 4 night and 4 night block...whereas leaving the single night isn't a good business decision for them.
As for the 9/13-9/21 dates, it's probably a different room.
I appreciate your feedback. There may well be some validity in what you are saying.