The data is from the theatre manager, not the pimply faced usher. Not that the manager might not have a few pimples. Also, spot checks are made on the theatres. These checks are made by independant companies paid by the studios. The checks include head counts and checking the ticket sales. There are also covert checks, where someone goes to see the movie and gets the count on that particular show in that theatre.
Well, now I know. I guess the book biz is more corrupt than the movie biz, LOL!