I'm highly skeptical of the claim that it sold out. Both Wii U SKUs are easily available in most of the US right now. Supplies were constrained initially, but that has more to do with scalpers buying up every preorder in anticipation of another Wii-style fad, which has clearly not happened. If you look at Wii U auctions on eBay, the few auctions that complete have been selling essentially for cost once eBay fees are factored in.

1.26 million Xbox 360s vs. 425,000 Wii U. Seems to me the Wii wouldn't have caught up even if they'd had more time and more inventory.

Since it is an improper comparison, what do you base your hypothesis on exactly?

It may have only been on shelves for 12 days, but people were free to preorder it all month, so you can't just divide the total sales by 12 and apply that to the other 18 days.

Also, a lot of people probably bought it for resale since it's the holiday season and the first new console in 6 years. Prices on Ebay are barely above retail though, so demand doesn't actually seem to be outpacing supply by very much.

If it had been on sale the entire month and (assuming) they maintained the same rate of sales as it did the 12 or so days it was available, it would have sold around 1.06M units.

I would imagine they could have made up the 200K difference their had been more inventory.