I think that its reasonable to assume that, a large portion of the people actually interested in console sales are going to be interested in gaming, and tech in general.
Thus... a higher percentage of PS3 owners here -- and a higher percentage of people who are likely to view the other consoles as "old news", and when asking themselves "which console would I run out to buy, given the choice", then don't choose the Wii or 360 (or PS2), despite the fact that both (all three) are fine consoles currently available at most retailers to the average joe.
Everyone who posts here has online access, or is a reasonably experienced online user. Many Wii, and some X360 owners, probably could care less about online, whereas its also likely that the same demographics that are interested in the PS3, are also internet-savvy.
In short. Site demographics. I know my own view of the 360 is somewhat biased, merely because I have a definate preference for the PS3 (I own all 3). I can only predict based on sales trends, and my knowledge of the industry.
The numbers I have listed in my sig are just guesswork, really, based on assumptions about why the 360 is doing so well (price point), and how long those sales bursts tend to sustain a product until the product is saturated to the demographics interested in whatever boon caused the recent surge. I'm guessing that most people don't even put that much effort into their own predictions -- its for fun.