From what we do know of his method of obtaining numbers, he gets information from retailers in order to give a rough idea of what the sales figures are. They weren't that far from the actual # in the grand scheme of things either, on top of that he would retroactively fix the numbers which makes older numbers a bit more reliable than your standard online source. It's not a site for credible numbers at all, but it's one of the better sites for, as you said, giving a general idea
Being a long time reader of this site (since 2007ish) I don't have a problem with the numbers being somewhat off. It is expected that a margin of error exist for a site that tries to gauge what a product sells in a market without having all of the data. However, the fact that number updates are so far behind and sporadic has been very detrimental to this site considering its very purpose is built around those numbers. Without that we have a forum, and while the community isn't terrible, there are better places for news and discussions. Here's hoping something is fixed soon, whether that be removing numbers altogether or catching up because it looks really dire right now.