What would be really useful to know is if Nintendo have access to Walmart numbers or not. We know they have a good relationship with WM, and neither NPD nor vgchartz has access to that data if I'm not mistaken.
If they do, then Nintendo should be taken as the more authoritative source. If they don't, then it's quite likely Nintendo were being deliberately conservative, as lower NPD results would hurt their PR.
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.