To be honest I couldn't care less what IGN thinks - they won't be around in 5 years the way they are going so their opinion counts for nothing in my eyes. What does bother me is the ignorance of some people in this very thread who still don't get it!
If we put out a figure saying that a game sold 600k units in a given period then we are not saying that the game sold exactly 600,000 units in that period. It is an estimate, based on much smaller data and extrapolated up. Think of it as a probability - when we publish a figure of 600k we are saying that we are 95% certain it has sold somewhere between 300k and 900k, 85% certain of it being between 400k and 800k and 70% between 500k and 700k and so on.
Take the 600k figure as the midpoint with a decaying range of probability around it, much like a standard probability bell curve:
For people to say something like "VGChartz is wrong" makes no sense. The word "wrong" doesn't make any sense in this context? There is no right or wrong.
If we claim 600k for a game and NPD says 635k then are we "wrong"? If they say 485k are we "wrong"? Not in my eyes, we are within a reasonable and expected margin of error - we'd be in the fat part of that bell curve above - especially when you consider that NPDs data itself will carry a 10-15 error margin. This is why people who say that have overtracked something because we have 790k and NPD has 740k just annoys me! Their figure represents anything between 650k and 850k and our figure represents anything between 700k and 900k so they line up just fine. A 50k difference out of 800k is a tiny and insignificant difference.
You have to start thinking of a data estimate as being a range of possible data not just one hard, absolute figure. I can't emphasise that point enough.
Now if we said a game sold 1.2m and in fact it only sold 50k then that would clearly be outside of any reasonable margin of error and right at one of the extreme ends of that bell curve above - which would clearly be unacceptable.
I'm afraid anyone who comes in with this idea of data being "right" or "wrong" has no clue what they are talking about and has obviously never studied maths (or math as you insist on calling it in the USA) to any decent level (such as Mr Schneider for example).