Thats an estimate.
And I dont think all retailers are counted for those numbers.
Those numbers are also reflecting a significant increase in sales.
So what happens if WiiU has the same type of percentage increase that it did from
October to November?
1) That's not an estimate (implying that the real NPD figure could be 200K-300K). We know, for a fact, that Wii U sold ~223K in the USA in November 2013. There's a reason why that 220K figure is in the OP.
2) A 342% increase is standard with Black Friday gains...remember, Nintendo has deals with retailers for incredible Black Friday deals...and Nintendo has invested a significant amount of money for USA November marketing. It was a given that the Wii U would have an increase in accordance with this.
3) NPD tracks (receives precise sales numbers from) every major USA retailer (both online retailers like Amazon and physical retailers like GameStop), totaling 90-95% of the video game market. Then, they estimate the remaining 5-10% to come to an extraordinarily accurate view of the current hardware market.
4) I know for a fact that Nintendo looks at NPD numbers very closely. That 223K number is what Nintendo is using as a gauge of the Wii U's health in the North American market. Nintendo doesn't operate under percentage increases, because a 342% increase of horrible sales...is still horrible sales. They're looking at total marketshare, software sales attach rate, historical marketshare, etc. The fact that 223K is way below the November sales of Nintendo's previous consoles (in their prime) demonstrates serious problems with the console.
5) Typical November gains in the USA market doesn't imply similar gains in December. It's very probable that Wii U will only manage gains up to 270K next month. Considering its health, I think it will be very unlikely for the console to undergo a 342% increase next month...but even if it does, you have to look at the console's position in the total market.