I don't think there's a pattern as in deliberate pattern. I think that the PS3/360 reviewers occasionally skew harsher than the general trend represented by Metacritic (in a flawed sort of way) while occasionally the Wii reviwers skew a little higher.
Now, assuming VG is using reviewers familiar with the platform this actually makes sense to me as I believe that:
a - general PS3/360 reviews tend to skew a little high generally, particularly for titles with official mag reviews, therefore I would expect a good, firm review to be a little lower than metacritic
b - general Wii reviews often seem to be by people who don't necessarily fully 'get' the platform or its titles, hence I'd expect a good, firm Wii review by someone who does to skew a little higher than metacritic.
Of course, maybe VG Wii reviwers are just big softies and the PS3/360 reviewers are all tough cookies.
Either way it's all a statistical accident. Probably. That was a slight joke.
Try to be reasonable... its easier than you think...