Gamasutra has an interesting theory about the Wii U console's low sales numbers for January. While NPD Group reported that only 57,000 Wii Us were sold in January, it is possible that, because NPD's number is based on consoles sold minus consoles returned, the extremely low number may be the result of returns from those who thought they could make a killing last year selling Wii Us.
Because the Wii had been so difficult to get when it launched, speculators had made good money reselling the console on eBay. People tried the same thing with the Wii U, buying several and attempting to sell them for double or more the retail price. But Nintendo had enough Wii Us on the shelves, and speculators were stuck with Wii Us they didn't want and couldn't sell.
Since big retail outlets tend to allow returns up to 60 or 90 days, the theory is a lot of speculators gave up in January and returned their Wii Us, forcing down sales figures. Tens of thousands of Wii Us may have been returned that month.
If true, this means that consumer sales of the console were somewhat lower in November and December than people had believed, and considerably higher in January. This doesn't mean the Wii U is selling well - 100,000 is still not great - but it means things aren't quite as bleak as they appear to be for Nintendo's latest machine.