That's a tale as old as time in Hollywood. Big stars have all kinds of clauses in their contracts to protect their brand image and ego. Even affects what you would assume to be minor stuff like posters. My personal favourite is from back in the 70's, where both Steve McQueen and Paul Newman share top billing for The Towering Inferno. McQueen's name is first, but Newman's is raised slightly higher. So if you read from left to right or top to bottom, a different person has 'top billing'. Stupid, pathetic but pretty funny.
Even Newman's image is raised slightly higher.