Once again, it's only my take on the subject, but letting people reserve a dining time doesn't mean the minute you arrive (especially at the busiest time) guarantee that you'll be seated the moment you walk through the door.
I see their reservation system as only a request for a dining time which cannot be guaranteed but they'll do their best to fulfill. Unlike a land based restaurant who will hold a table empty, the ships tables are in use prior to you arriving & considering the number of people eating there every night makes it impossible to hold empty.