Dang, I wasn't aware that Witcher 2 was USK 16, I was going by the original game's USK 18 rating. Oh well, guess a USK 16 rating seems legitimate in case of Witcher 3 as well, then.

I'm wondering, though, how Witcher 2 could end up with a different rating than the original game. Seems to be just another case of sheer inconsistency when it comes to the USK's work.

It just developed over the years. Our government indexed River Raid in the 80s, because this game was "promoting violence".
http://en.wikipedia.org/wiki/River_Raid

In the last years some cut german versions are just because of the publishers cut the game even before the give it to the USK. For example New Vegas and Sniper Elite V2. They removed the Kill-cam from the game. Later they trust tried to give the uncut version to the USK and just got a rating, even for the Killing Hitler DLC...