Some folks have a strange idea about the Steam concurrent-user numbers. This is the number of people online in the Steam network multiplayer games--it is not by any means the absolute number of people playing the game...;) Single-play isn't counted here, nor should it be. The Witcher 3 had no Steam copy protection added and many, many more people played the game single-play than multiplayer--I run the game from its executable and Steamworks doesn't even have to be running to play the game (a lot of Steam games have no copy protection, actually.) So...there's no way that the concurrent number even comes close to the number of people playing the game. I guess some people thought that's what the concurrent number means, somehow. I've got several AAA games that run right from their exe's, completely by-passing Steamworks. This is by design, of course, because the game developers decide whether their games will include DRM or not--even though I'm certain many Steam users couldn't tell you which of their games required Steamworks to be running and which don't. Additionally, of course, people sometimes play in offline mode--that's single-play, too, and of course in offline mode Steam has no way to know who is playing what game, etc. As such, the number of people playing online in the Steam network is sure to ramp up and down continuously. We know they've sold ~14M copies to date--so obviously most people playing the game aren't playing on the online Steam multiplayer version of the game.
It is well known that I don't make mistakes--so, if you should happen across an error in something I have written, you can be confident in the fact that *I* did not write it.