Why use the word shifted? Seems not the best word. Wouldn’t it be sold?
My guess is that’s either a typo and was supposed to be “shipped”, or that they deliberately chose the word shifted (“to exchange for or replace by another”) in order to combine the numbers of sales and repairs into one statistic so it looks bigger. After Nintendo was mass-sued about the poor quality of their C-sticks, they were ordered to replace / repair any stick drift issues for free even outside of the warranty period, and people naturally used that feature.
I for one had to send my joycons in for repair six times since buying the switch, so in that statistic my hardware would have been sold once, but “shifted” seven times.
It is a common term. I am not sure why it is used. Possibly because the consoles were not sold to end users but shipped to retailers. Some (small portion) is sitting on store shelves.
The Switch seems to still be selling fairly well, so there is not yet urgency to move on to the next console. Having said that, I imagine it will take a while to roll out that console, even if they are already manufacturing them in bulk (though if that was the case, I would have expected to see some more leaks).