cast order problem

Recently, I have realized something interesting about the cast order of a particular film. When I attempted to edit the cast order of the film La regle du jeu (1939), I saw that the existing order was like this: Nora Gregor (1), Paulette Dubos (2), Mila Parely (3) and so on. However, in the film's page (not at the editing section but at the usual page, the way everybody sees it), the top three listed actors are Marcel Dalio, Nora Gregor and Paulette Dubos. Then, I went back to the editing section and saw that the order written for Marcel Dalio there was 8. In this case, why does he appear as the top billed actor on the film's page? There is no way for a user to change that. Who decides which actor gets the top billing, if the actor ordered at number 8 can appear as the top-billed actor on the film's page?