The thing is though you can't really compare NPD and VGC as they are quite different.
For example
From 3rd jan to 27th nov 2010 VGC has PS3 at 4.131m for americas. X0.89 and you get 3.67m for US.
NPD have up to the same period
Jan - 276k, Feb - 360k, March - 313k, April - 180k, May - 154k, June - 304k, July - 214k, August - 226k, September - 312k, October - 250k, November - 530k. Which is a total of 3.119m for US.
Meaning VGC has PS3 551k higher for the same period then NPD has it at. That is before you get the december numbers.
Now i imagine if it was the other way around and VGC had it 550k lower then what NPD had it at the calls for undertrackedz would be quite deafening ;)