It is standard for all hotels and resorts, Disney or no Disney, to not assign hotel rooms until you or your party arrives on arrival day. Hotels keep a running tally of open rooms, room service cleaning rooms, etc. on pretty much a real-time basis. Once you booked the hotel with whatever amenities -- queen beds, king beds, sleeper-sofa, suite -- that is recorded and when you arrive a room matching your wants and needs (unless it is for a special needs person or elderly), will be assigned.
The room or rooms are assigned instantly when you check in.