Sales figures on VGChartz are based on estimates extrapolated from small retail samples. When Brett Walton (the owner of the site) was asked he gave the following answer:

"The methodology we use for all of our charts in all regions is the same and our data is arrived at by a combination of the following:

- Sampled direct sell-through data
- Industry knowledge and experience - applying past trends in terms of marketshares, regional breakdowns, casual vs hardcore and so on 
- plenty of statistical analysis, regression calculations, market projections
- Contact with industry figures - buy-side analysts (such as Pachter / Divnich), sell-side analysts who work with us on specific products / projects, manufacturers who work with us to project sales of their key titles
- Retail checks - we have a team who talk to stores and estimate shipment figures for low-stock and hard to find items which we struggle to track with our normal data samples.

Exactly how we get from these various sources of data to final figures differs from game to game and console to console and our exact methodologies are confidential for obvious reasons."

those people and boards think its biased anyway, but why do those people matter??