35 hours is good for an RPG and you are getting your money's worth for that. FOV might should another 10-15 hours onto that and that will give you either 45-50 hours of gameplay which is typical of most RPG's. Still, when the back-cover is saying "one hundred hours" then you expect one hundred hours. However I guess Larian said that statement based on the average finish time of testers and based on the comments in thread, most people got 100 hours out of the game in one play-through. I guess me and you are just faster at games than other people.
Divinity 2 isn't the only game I've played where I've beat it quicker than the time it's said to last for. Dragon's Dogma was supposed to be a 200 hour game and I beat it in 70 hours. Dragon Age: Origins was supposed to be a 70 hour game and I beat it in 30 hours first go (my quickest time is 17 hours).
I've learned to realize that when someone says "100 hours in content" it's either 40 or 30 for me. Difficulty does increase the time though. Halo games on normal difficulty are around 7-8 hours long first go. Play on Legendary and each level will take around 1-2 hours giving you 15-17 hours in total by the end. I think my nightmare difficulty playthrough of DKS will end at the counter of 55 hours (as opposed to the 38 hours I got on normal difficulty first go but it should be noted that I had previously played through the original game - before DKS - twice and thus I knew how everything could be done).