Pretty sure it's easier to be accurate when you're shipping way less anyway. That is, the percentage of consoles sitting on shelves compared to consoles sold would probably be a better indicator, and that should make it more even. That said, Nintendo seems very accurate.
Also, hype. If MS overships, they can brag with their shipping numbers, which should be good for business. And all those units are guaranteed to sell in a reasonable amount of time, so there's absolutely no problem there.