They were checking your S&S card at the entrance to be sure you were "Anytime" or they sent people to the buffet.
As with many things on cruises, I'll bet this has more to do with the dining room manager. They might also allow a traditional diner to use anytime if they show up at a slow period and not at peak time. It is also possible to use anytime and reserve a table for the same time every night - thus you can have your own customized "assigned" seating.
Personally, if I wanted to skip early seating one night I'd want to try an alternate restaurant or the buffet dinner.