Yeah, I double checked GOG Galaxy and 100 seems pretty low. Last time I played it through (using autoloot mods to speed up the boring going through every trunk, bag and sack I see and already having completed it before) still took me 190 hours. The game is huge and due to the multiple branches you literally can't see everything in one play through. In fact that's one of the main reasons I wanted to replay the whole series, I wanted to see what would happen if I went with Iorveth rather than Vernon in the second game.
The simple fact is that CDProjectRed are terrible at judging how long their games will take to play. There's a really good (but massively full of spoilers) documentary on the making of the series here
and in one of the episodes they talk about how they thought that the game would be too short at only 60 hours. Funnily enough that's about the time it took me to play the two expansions, not the main game.