I'm not doing Americas to US lol. That's just a standard margin of error loosely based on overall tracking accuracy. Should VGC numbers be below NPD ones I'd put in a 10% margin all the same.
That's kind of pointless since margin of error works both ways and for both sets of data. You could just as easily say, for example, the Wii is overtracked by 146k by taking the upper limit of VGC and lower limit of NPD (assuming a 10% margin of error for them as well).