Different films come with different deals from the distributor, usually determined by the expected popularity of the film. In the case of a film like "The Passion" which is expected to be strong out of the gate and have a relatively long and consistent run, the theatre pays a larger percentage of ticket sales back to the distributor for a longer period of time. The theater makes virtually all its money at the extortion (concession) stand. As sales are expected to drop off the theatre gets a larger percentage of the gate as an incentive to keep it running. For a film not expected to do especially well, or one nearing the end of it's run, the theater makes more, plus of course whatever they can mug you for at the concession stand.
By tabulating the sales against another film than The Passion, the theater keeps the gate receipt rather than having it go to Gibson.