Remember, I doubt there are more than 50k (if that) sitting around in Japan, meaning that Microsoft can have decent supply the world over with a million a lot easier than Nintendo can.
Also, just because they have similar "stock" doesn't mean they would have similar supply issues. Look at the Wii sales WW (or just NA and Others if you wish) for the last few months, and compare them to the 360s WW (or NA+Others, since 360 in Japan is... you know), and you'll see that the Wii needs more stock to have healthy supply.
Take for example if the Wii has (according to Nintendo and minus VGChartz) about 400k in stock in NA. That would be about enough for two weeks of sales (if that). That's a supply issue. While 400k for the 360 in NA would be enough for about a months worth of sales (5 weeks plus extra).
It's debatable whether the 360 was having some supply issues earlier this year, but since the 360 actually beat the PS3 in March (according to VgChartz AND NPD), after being beaten in Jan and Feb (according to NPD), it's not hard to say that there might have been a few supply issues.
Basically, I think it's good as it is. Without Japan, and lower sales than Wii everywhere else, it doesn't surprise me that they'd have similar stock WW that's "unsold."
Edit: I'd also like to point out that a million Wiis in stores WW would only last 2 weeks plus a little of a third week, while a million 360s in stores WW would last somewhere around 6 fulls weeks, plus most of a 7th week. And perhaps retailers are less willing to accept large amounts of them, since Microsoft stuffed the channel last year, and without Vista, Microsoft has no way to force units into the channel?
