There are a few thing to consider
1)The economy improved in 2017. Q2 and 3 saw 3 percent growth. Q4 will also be good. It's the most consistent year since the great recession started
2)PS4 Pro inflates the numbers through double counting. If I own a PS4 and buy a PS4 Pro, I'm adding two sales but I'm one customer. I should note this is happening with XBox One and will happen to the Switch as well
3)Good games in the early part of the year helped.
The stars were aligned for a good year. This year would have been the decline (assuming Dec isn't catastrophic of course) and this is reflected in Sony projecting fewer shipments. Originally it was only 18 million and Sony upped it to 19 million. 2018 will see a decline. While the economy will still be strong, PS4 Pro numbers will decline (due to time) and the selection of games may not be as strong.
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Known as Smashchu in a former life