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.