Part of the reason sites like NeoGaf ban Vgchartz numbers is because they aren't set in stone, Bret reserves the right to change figures after the fact based on information from stores that report less frequently, official company figures etc. This of course causes huge fanboy issues on a number of sites when a game is annouced to have sold 1 million first week but later the estimate gets scaled back to 900K. While the error on this type of change is only 10% it tends to create flamewars and a mistrust of Vgchartz for people who take estimates too seriously.
As far as numbers from obscure counties, obbscure countries tend to buy very few games compared to larger counties and so even if a source of data cannot be obtained they can be accounted for in estimations based off the purchasing habbits of other countries.
Actually the reason Neogaf bans this site is because these numbers could of be on neogaf and they refused...
and now they're jealous.