Xbox360 better than PS3 games?

Games like GTA IV, Tony Hawk Project 8, Burnout Paradise, and other games that are both avilable for 360 and PS3, but why does Xbox 360 gets better ratings than PS3 even though they are the same game? One of my friend says that Halo 3 came out on 360 since Halo game is popular. I dont get it! Is my friend right or wrong?