Someone explains this to me : FFXIII outsells Mass Effect 2 by 1'632'195 in the west

[A203D]

Could the OP explain this to us:

FF13: May, 2010 6.2 mil copies shipped. January 2012: 6.52 copies sold.

FF13-2: awaiting western launch, underperformed FF13 launch in Japan by about 1 mil copies.

Skyrim: Released November 2011. Decmber 2011: 10 mil copies shipped!