This is a great question and I think Ian nailed it - but I also wanted to bring up the classic story of The Towering Inferno:
Stars Paul Newman and Steve McQueen apparently argued intensely over who should get top billing. In the end the producers settled for a compromise: reading the film poster (which is reproduced as the DVD cover) top to bottom, Paul Newman is first, i.e. higher, or "top" billing. But reading left to right, Steve McQueen is first. The same applies to their photographs either side of the main artwork, McQueen on the left but Newman (marginally) higher up. Ironically, this billing format was an issue when McQueen was considered for a role in "Butch Cassidy and the Sundance Kid."
Taken from MovieMistakes.com.