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.
