Here's some proof for you to ignore.
The first and obvious one:
They often have no sources to back up their sales figures. if there is no source, then where did it come from? A birdie?
Second.
They themselves admit on their about us page, that they make, quote:
"ground-breaking weekly charts based on estimates from retail partners and passive consumer surveys." They make GUESSES based on partial info. Not official info from the dev. Of course when official numbers DO come out, they are quick to change their charts to match them.
Third.
Their accuracy, while sometimes impressive for guesswork, is still not 100% accurate. which means that while it might be easier to estimate sales for a AAA best seller game or console, when you get to smaller games, or not as successful games, it becomes much harder to make accurate guesses.
Also, considering some of their sources for sales, (retailers and surveys) You can't help but see why they would be inaccurate. Especially retailers, who would prefer their numbers be higher or lower, and therefore can lead to inconsistencies. And even the big stuff is not 100%
here's a legit copypaste of their self admitted methodology, right from their site.
VGChartz Methodology
Data-Collection Methodology
All sales estimates on VGChartz are arrived at via a number of proprietrary and ever-developing methods:
Passively polling end users to find out what games they are currently purchasing and playing
Polling retail partners to find out what games and hardware they are selling
Using statistical trend fitting and historical data for similar games
Studying resell prices to determine consumer demand and inventory levels
Consulting with publishers and manufacturers to find out how many units they are introducing into the channel
do note that last one. they might contact publishers not for sales numbers, but how many units were introduced into the system.