They usually give the table request to those who book first. Good reason for booking early, if this is important to you. We book early and have always have our request filled.
The reason you saw tables for two empty are because some nights people eat at the Lido or alternative restaurant. We know of one couple who asked every night, if there was a table for two available, and they did get one nearly every time, sitting in the place of someone who had reservations in the alternative restaurant. Of course, they never got to sit at the same table twice, but they got a table for two whenever it was available.