Sure, but:
Number of unit sold/number of days on the market: X1 wins... therefore, it's sales pace is higher. Therefore, it's the fastest selling console in NA.
Not that it's a big deal, and obviously it's preferable to have sold more units world-wide (which Sony did), but still - the fact of the matter is, X1's sales pace is higher than PS4's in the US. Not sure why it bothers some people here... it's just a fact.
Cool. PS4 still managed to be the best selling next-gen console of 2013 in December though with total amount still greater than the Xbox One's even with supply constraints. Wonder how much the total amount gap would widen if supply was not constrained.