By this logic, high budget, PC exclusive "hard core" oriented titles (and by this I only mean no Sims or Barbie) should do noticeably better on PC than the multiplatform games do.
When was the last time there was a high budget, PC exclusive hardcore game? Crysis? I hardly consider that hardcore. It had regenerating health, was a breeze on Delta difficulty and only lasted ten hours. Sins of a Solar Empire is a hardcore game but it isn't exactly high-budget.
As for Bioshock and CoD4, I don't know how well those sold on the PC but since they were multiplatform, they got a hell of a lot more marketing than any PC-exclusive would ever get. Orange Box was multiplatform too but still sold better on the PC because it was actually designed for PC gamers unlike every other multiplatform title.
And yes, we know that none of these arguments are going to change anything. Publishers believe what they want to believe. That doesn't mean we can't stop criticizing them for it.