Well, this is how a game with random hit chances across the whole spectrum plays out. There is nothing wrong with the random number generation in the game on a technical level, the human brain just isn’t particularly strong at working with chance and probability. Try playing poker for money if you want to get real mad at chances ;)
As mrbunnyban suggested, a possible alternative on a design level would be like Pillars of Eternity handles it; adding more outcomes inbetween the binary choices of ‘hit’ and ‘no hit’ would even out the damage sooner, i.e. require a smaller sample size on average for the actual damage to approach expected damage numbers and reduce the element of luck. However, it would also diminish the impact that a single hit has, both in damage numbers and emotionally, since you’re kind of always hitting for small amounts of damage. This would change the feel of combat (and ultimately the game) quite a bit, away from the generally fast and deadly style we want it to have, more towards applying constant damage over time. It’s not perfect either way, but we’ll probably stick with the system we have.