Wait. I thought SS neared the 5M ... Oh guess I was wrong,thanks !
You were probably remembering the old VGC estimates. Hugely overestimated Skyward Sword. I think something around 4.4M originally.
It then got adjusted down to around 4M but even that was not enough. Official shipment was only 3.67M lol
Guess the bad E3 showing and the Motion Plus requirement really hurt its sales, also it is a very divisive game in the franchise.