Theres multiple reasons why I think this is true.
If the PS3 had been able to fill more demand, then it would have started off with a nice base of 1.5-2million in the first few months. That would have kept a great deal of those exclusive that Xbox took, including BioShock. Also, this would have stopped the Xbox from taking so many sales
I know last winter when I couldn't get a PS3, I went out and got an Xbox. The was cheaper, it had Gears of War coming out, and it was readily available.
And while the Xbox did have shortages as well, it didnt have competition to take away sales. Thats the big difference. You can see proof of this as even this year the Xbox will have trouble matching last years holiday sales.







