These things take time... By the end of 2017, I seriously doubt the Wii U will have had any trouble out-selling lifetime sales of Xbox or GCN. It'll probably be at 50-55 million globally at least.
Launch numbers are just that. Launch numbers. It doesn't factor in whether there is strong --or weak-- software support, how much strong software the console ends up getting throughout it's five or six years on the market, when price drops hit, how much it's bundled, and how well it sells during the holidays.