Why isn't VGchartz credible for games/console sales data?

[krjcook]

Whenever someone post sales of a particular console or game with it's source being VGchartz someone always leaves a comment about it's credibility, like the sales data is over exaggerated, such as...

"when I saw VG I laughed", "vgchartz isn't a source".
Now these quotes aren't word for word but along the lines of what some people are saying.

Why isn't VG creditable? Isn't it's data very close?

Whenever someone post sales of a particular console or game with it's source being VGchartz someone always leaves a comment about it's credibility, like the sales data is over exaggerated, such as... "when I saw VG I laughed", "vgchartz isn't a source". Now these quotes aren't word for word but along the lines of what some people are saying. Why isn't VG creditable? Isn't it's data very close along the lines?