Here is the problems
Xbox One - While most of its inventory (to where you have 1.7m on shelves from the last shipment) is selling out. Forza bundles, Xbox One Sku, etc all sold out in Best buy, gamestop, amazon, etc If that is true then the 1.7m gap should be smaller. Now with another shipment which is looking to be the Call of Duty and Assassin's Creed bundle for majority has taken over. That would signify that last quarter would be undertracked, my guess in Europe and RoW. Id wager by around 300k at the very least.
PS4 - The PS4 is the most obvious since they give actual sold through numbers. Certainly is undertracked and i dont see how you can argue against official numbers
WiiU - If we are to believe WiiU is correct then we would have heard about WiiU shortages based on shipment number which we most certainly didnt. While they dont stuff the channel as much as Sony/MSFT, i highly doubt that there is that little on store shelves. Overtracked