Crap! I think this is the most inaccurate VGC vs. NPD I've ever seen: everything undertracked and all of them by a "need to adjust" amount. And it's the biggest proportionality difference between PS3 and 360 I can recall.
The US data sources being used obviously didn't reasonably represent the US market. Question is whether the non-representative sampling is merely a blip or a there's now an underlying sampling error bias.
Over the years I've been of the opinion that in aggregate VGC cf NPD has been within a reasonable margin so this might just be one of those 1% occasions where the sampling does not reasonably reflect the reality (not that NPD is complete reality either as it's merely a much bigger sample, not a total head count).
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