That's cause if someone cancels or maybe even upgrades and changes to a higher category that cabin they had will become available and show up in inventory. They are probably being grabbed by people like you watching to book a last minute cabin or a TA grabbing it for a client way before a Royal rep goes through their waitlist. I wouldn't ever wait list a cabin if I were looking to book last minute...I would just keep checking myself and if I see something pop up grab it.