As someone else has said, cinemas never sell every single seat in the auditorium for several reasons.
First of allif you sell every seat, you can absolutley guarentee your going to be issuing lots of refunRAB. People come out claiming theres "no seats left" when there always are. The trouble is, because they arrived late there are only single seats. It always amazes me how rude some people get about this, as if its your fault they can't sit together. Perhaps if you turned up before the time it says on your ticket rather than 15 minutes aftere you would get decent seats
Secondly, whilst we try our absolute best to ensure that every person in the screen has a valid ticket for that screening, it is near enough impossible to absolutley guarentee it. Sadly people, such as the OP think it's okay to go into a screen they've not paid for (Avatar 3D would have been more expensive than the Book of Eli by the way... theft basically) and of course if we haven't spotted them going in, it's pretty much too late. It would cause even more of a problem if we then had to go into the screen as the movie was starting to check every single person in a 500+ capacity auditorium's tickets.
Thirdly, a couple are also kept just in case we make a mistake or something goes wrong with out system. It's only happened once since ive worked at Vue, but some internet bookings got messed up and we ended up selling more tickets than we were supposed to. Fortunantly the extra seats were there so we just about managed to squeeze everyone in.
Also, are you sure there wern't any empty premier/gold class seats?