Not to mention that PS3 3rd-party costs more so they need to sell more. Why do you think there are so few PS3 3rd-party exclusives to date? They're all on or going to be on 360 at the least. I can't even name any already out off the top of my head except maybe RR7 which I heard was just an upgraded port of RR6 on 360.
edit: wow just noticed Sonic Wii outsold VF5 on PS3, that's interesting wonder if it holds up.