NPD numbers aren't true either though. NPD is merely a tracker as well.
Sure, they may have more coverage than VGC but they still don't have 100% coverage so they still aren't 100% accurate.
NPD are numbers from retailers and distributors with a margin of error. VGChartz numbers are straight up guesses with less tangible data. There's a huge degree of difference.
If you call VGChartz numbers guesses, than you must also call NPD numbers guesses. NPD might have more data, but it's not that different. Also, VGChartz does adjust it's numbers when more data is available, so the older the numbers, or totals for something that has been selling a long time the more accurate.