And that's the problem here, they don't present those numbers as estimations. To make matters worse, they don't even update their numbers when official sources come out about it. For instance,
Pokémon Black and White has sold 15.58m as of March 31st 2014,
VGcharts has it at 14.95m as of this
November, which is lower then the official source has given. So not only is their estimation wrong, they didn't bother to fix it and continue to present the wrong numbers, which leads to them giving out misinformation.
That's why they get dismissed, if the numbers they present are not honest, updated and unsupported, you can't place a lot of faith with them. Which is why if you want accurate sales data, people will tell you it's better to get another source then rely on VGcharts.