Yup. It used to be something around ~60% of the market NPD had access too, but they had to extrapolate Wal Mart, which was a biggie. Even then they were extremely sophisticated about that, using panels of Wal Mart consumers. They estimate sales for a lot more than video games.
But some time ago NPD got access to Wal Mart data again, so at this point AFAIK there's very little error margin in their numbers at all.
Chartz OTOH is just some guy (Brett Walton/ioi), and nowdays probably just "staff", more or less, lol. He does not have legitimate access to any numbers. I know because a few years ago when I mentioned working in retail on their forums, I received a PM from an admin asking if I could provide any sales data. Such tactics are obviously illegal among other things.
The idea that somehow you could decently estimate sales from non-direct methods, for example, online leader boards (for software sales), achievements, historic knowledge of different markets/market share, so forth and so on, along with a smattering of (technically illegal) retail sampling, which is what vgchartz purports to do, seemed to me to be a little promising. However years later, Vgchartz before the fact NPD numbers are still basically no more accurate than guessing.
At least for some markets, like maybe UK, France, Germany, you do get occasional sales reports (thinking mostly of hardware here) in a reputable newspaper or the like. And I checked one time, Chartz does retrofit their EU numbers, because the reports match their numbers pretty closely at least I think it was France I checked once.
But your point is still valid, for any given non US/Japan market there's no telling how accurate their past data are.
