Well, "Sony as a first party publisher on retail disk $60 games" might have more exclusives available compared to "Microsoft Studios first party publisher on retail $60 games". But outside of that small number, the Xbox actually has substantially more exclusives to choose from. So the whole 1 vs. 10 thing here doesn't work.
I have to say it is all in marketing. Our local Best Buy has one FULL aisle more of Xbox games and accessories than the PS3 does. Microsoft, as well as 3rd party accessories seem to be supporting the Xbox a bit more in product and product placement. If MS is dumping millions on shelf space, that alone is going to sway consumers. They will assume the demand is for the green packaging.

