why did mass effect 2 only sell 2.3m on 360?

[Carl]

hudsoniscool said:

bioware said the mass effect series has sold 7 million copies. vgchartz has them at 5.6 million. so unless there has been 1.4 million digital downloads or we have it undertraked.

Americas sales for the PC version of Mass Effect and Mass Effect 2 are missing from VGC data, along with whatever it sold DD.