The more compelling evidence, anyway, is that in order to have sold more on the PC than the 360 it would mean that it was the best selling platform/game combination, MW3 excluded, since November. Do you really think that the PC has the single best selling game in that time?
It's possible. I assume you're basing your argument on NPD numbers, right? The problem with that is that the NPD only covers U.S. retail (and not even every U.S. retail store). This paints a pretty misleading picture of the market. There has yet to be any reliable source for digital sale figures so it's much harder to estimate how well Skyrim has done on PC.