It’s just RNG.
If you roll dice you’ll find that they quite often do things that don’t feel right, because randomness feels weird to us humans.
It’s not possible to use a single thrall to be able to see how the extra % chance works, they only level up 20 times and a lot of weird things can happen with only 20 rolls of the dice.
Sometimes you get lucky on stat rolls, sometimes you don’t, there’s nothing more mysterious than simple RNG happening.