Also, a lot of 360 exclusives seem to focus on multiplayer like Halo, Gears, L4D, etc. While many of the PS3's games focus on single-player Uncharted, Metal Gear Solid, God of War, etc.
And of course, it's common knowledge that single-player games are rented and sold used lot more than multiplayer games. According to BenVtigger, the used-game market is huge. He claims that the Borderlands install-base is huge even though sales aren't that high. That would make sense considering the huge pre-order numbers for Borderlands 2.
With that in mind, it wouldn't surprise me if many of those single-player focused PS3 exclusives actually had install-bases much larger than those numbers suggest.
So that, along with better marketing, more appealing titles, and a much larger install-base in North America explains a lot.
