I think Reach's problem is he wants the gap between 360 and PS3 to be reset back to before the Slim's release. Sorry, but it's just not going to happen. Also the 3.8 mil stated by Sony is an estimate of retail sales to customer. Just check out the orignal source as posted by Key2001. It also states that it exceeds 3.8 mil, so VGC look like they are right on the money.
When I was new here I was kinda ticked at the numbers, because I didn't know that companies gave shipment numbers, not retail sale numbers. But once I figured that out, I love this site. I haven't seen where they have been terribly wrong, and need a huge adjustment of say, oh I don't know, 1.4 mil.
As far as NPD goes, they do the same thing this site must, estimate. As people have stated before, Wal-mart and some other stores don't release weekly or monthly info anymore. So they have to estimate those sales they have no info for. Honestly, I think most people go with NPD is because it's been around longer, and in doing so has become the industry standard. Though that has started to change, as Guinness used VGC data for their recent video game record book. Also, I have never heard any good reason as to why NPD would be more accurate with their guesses.
Also Reach, I highly doubt that when Sony releases its official Q3 numbers (those will be shipped, too) You'll be screaming for VGC to correct the number to make them higher. On a side note, I find it kinda funny that while Sony and Nintendo are touting estimated actual retail sales, MS is relying on shipped numbers to make it seem more impressive. But judging by Reach and Algorith's posts, I guess it worked.