In an article at "The Industry Standard", Net Applications apparently admit that their numbers are skewed. More specifically, they admit that they are skewed towards certain regions of the world (US-centric, anyone?). In such an amazing moment of honesty, I wonder why they didn't also admit to actively editing their own statistics.
Update: Their claim to be skewed towards Europe doesn't really match XiTi Monitor's statistics for Europe. This would indicate that Net Applications is heavily skewed towards the US.