It's possible they use a queue system similar to the one used for concerts. Once you select a room/ticket, it gets held for a few minutes while you check out (a

It's possible they use a queue system similar to the one used for concerts. Once you select a room/ticket, it gets held for a few minutes while you check out (and taken off the public market). So you can have a situation where there's no room, but someone else fails to complete the reservation and it becomes available again.