The opening weekend for a film says a lot about its probable success, and in terms of earnings over that weekend, the film in a franchise with the highest earnings is most likely to be the eighth.
That means when you take the opening weekends for each film in the top major franchises and find an average for every entry in a series - the average of Attack of the Clones and Chamber of Secrets gives film 2, for example - it's actually the eighth film that comes out on top - presumably if a franchise can make it to eight films, the public is pretty keen to see it.