In the United States two seatings sometimes means that they may not take any 7:00 or 7:30 seatings because it is hard to get anyone to eat at 5:00 PM and then also hard to get anyone to eat at 9:30 PM meaning a 7:30 reservation would only allow the table to be used once and not twice.

Back when in the restaurant business, almost every one wanted a 7:30 reservation jamming up the kitchen unless the restaurant stopped taking reservations at that time.

A good reservation person would only say we have these times available.
Also with opentable, you may not find a certain open but may get the time you want by calling.