Ok then it is 4 weeks for both January and February. But I still don't see why in average, people estimate +10-15% for Feb compared to Jan.
Well, like you said, 35$ Gift card; but also boosts from the direct videos. The 35$ Gift card was pushing the console very much both on Amazon and Gamestop, but it didn't push out of the charts the versions without the gift card, either. On Amazon for instance, all 3 SKU were selling faster than any PS4 or XBO SKU despite only one having a promotion.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454