When that happens, it can be a sold out restaurant (unlikely), a group event (also not likely), or a pop up.
In the past, you didn't know about pop up dates until you boarded. Darn you, internet for ruining surprises!
FWIW, Le Cirque was one of the best pop ups. We dined at the original Le Cirque and Le Cirque 2000. The Pinnacle Grill version wasn't quite the same, but it cost less than the appetizers at the NYC locations.