The issue is that the sources ioi gets data from probably can't track alot of the more "general" american retail stores like K-Mart, Wal-Mart, and such (atleast in my mere opinion).
That's why DS sales were way off too. When you get a more basic, huge retail store like Walmart, their sales can come and go in droves....In this case, it seems like the sources ioi uses had a typical, easy-to-see holiday boost for the Wii and PS3, as these stores give a great "sample" that easily compares to NPD. Wii and PS3 sales are usually not too far off in the US.
On the other hand, the DS and X360 are usually more difficult (as well as the PS2). This is probably because sales vary greatly on when, and where sales occur. In the case of the DS, it wouldn't take much for people to rush out and buy it at a Wal-Mart moreso than anywhere else. When I was at Wal-Mart on Black Friday, tons of kids hovered around the cases. If ioi couldn't track Wal-Mart, or other general stores, he'd have the most to lose in terms of accuracy from them.
But that's not ioi's fault. He needs to do whatever he can to get ahold of a bigger sample size though, preferably with more "general" retailers like Toys R Us, Wal-Mart, Target, and such.
Back from the dead, I'm afraid.
