It's simple.

A game that appeals equally to 360/PS3 owners will sell more on 360 due to larger install base.

A game that appeals more to 360 owners (that thing called demographics) will sell way better on 360 due to the install base added to higher interest from 360 owners

A game that appeals more to PS3 owners will sell slightly better on PS3 as the higher interest from PS3 owners will be offset to an extent by the 360 install base advantage.

The best answer is the 360 has a much larger userbase than the PS3. Some multi-plats sell less on the Wii because they aren't very good Wii games. Just read any of the Nintendo threads and you will see what I am referring to. Dead Rising would be a good one to look at.

I think a better question for your thread is why doesn't the 360 outsell the PS3 version all of the time?

Well

1st your statement is not true : some games do sell better on PS3 than on XboX 360 (RE 5, SF4 for instance)

2nd : when it comes only to USA, Xbox360 has a far larger user base and has been on the market for a YEAR more than PS3

3rd : when it comes to Wii, lot of games DO sell far better than on so called HD consoles : Guitar Hero 3, Guitar Hero World Tour, Lego Star Wars, Lego Indiana Jones (non bundled), Shaun White, Sonic Unleashed, Sega SuperStar Tennis, and most anime games just for exemple...