Sales are factual data thats actually recorded by official databases, using nexgen wars as the basis for this is horribly innaccurate for all consoles. PS3 under one million units worldwide? Wii at only 2 million units after releasing in all major territories? That site is horribly bogus.
Besides, its not like once the actual data is released for the month of december, it will include the time between the end of December and the time the data is released, its never worked that way.