I was curious, so I started a new game and tracked the results of the displayed rolls. At the time of this post I got so far: Character: Warlock (The Great Old One) Companions: Shadowheart, Astarion, Lae'zel Progress: Cleared Ruins, Visited Grove (Killed Harpies), Cleared Blighted Village, Cleared Spiders (the series of "80"s at the end is from cheesing the spider queen with Astarion^^)
But as far as I can see it seems that the probabilities/rolls in the game actually work out. The error charts show that very well. The error between actual and expected rolls gets smaller with an increasing number of rolls.