It would be complicated to explain, and it´s not my expertise, but I have a friend that is modding the game. From what he and the modders of the WoTR discord say, If you take a look at the code the formulas for auto-battle are purely mathematical, with a slight bias to favor the player. It does not make an "actual simulation" of the combat.
It happens the same as the RNG, it causes the same perception of unfair randomness. I´ve seen plenty of posts and threads in this and other games (Bg3, pf, wotr, Solasta, phoenix point, etc) stating that the dice is bugged, it´s not just or messes with the players... when it's simply a pure d20 dice. There are games that include the option to load the dice or add less randomness to the dice, or simply use crooked dice to avoid too many misses in a row, like XCOM.
