Realistically speaking, the accuracy of NPD is probably in the range of (+/-)2.5% 19 times out of 20 whereas vgcharts is probably in the range of(+/-) 10% 9 times out of 10 ... What this means is that vgcharts will always end up with a few consoles which are quite a bit different than NPDs data but (on the whole) should get somewhat close and be very usefull in determining the approximate ratio of how two consoles are selling or how games are selling.
Honestly ioi doesn't have the resources to track data like NPD does so you're always going to have to accept a certain ammount of error ...
The fact is that this really shouldn't matter all that much, most people are hear to discuss more about how consoles are selling in comparison to each other (and historical sales) and the charts ioi gives us are plenty good enough.
