How accurate is VGChartz when it comes to small sales data?

[MarkNL]

Hi, my first post so short introduction:
Mark from Holland, near The Hague. PS1 and PS2 collector, RPG game player and student Informatics. Came across this site while searching for the sales numbers for some PS2 games. So, that leads directly to a few questions I have. Probably some of you are able to answer them.

So, what is the source of all this information? I'm mainly interested in the sales numbers from the PS2 era.
I know it's very difficult, but let's take an example I stumbled across: VGZChart says there are about 0.02M (like 20.000) copies of Valkyrie Profile 2 sold in Europe. When I open the financial report of Square Enix, I see there are at least 170.000 sold in one year. That's a pretty reliable source if you ask me. Where does this very big difference comes from?

Second question: is there a way to get the sales amounts myself? Is it the effort of mailing companies worth?

Thanks in advance,
Mark