I suspect it may be down to footwork / timing errors, due to being unaccustomed to the slower speed. I know when I go from fastest speed to medium speed, I tend to land sloppy hits due to turning too much and moving too early. I expect going from a faster 1h to a slower 1h would cause the same kinds of issues.
Its the warband RNG, damage is random as fuck (its stupid)
Against somebody with no head armor, the damage variation due to randomness should only be 10% (you can do between 90% and 100% of your raw damage). It's when armor comes into play that things start to get really random (soak and reduce can vary anywhere from 45% to 100%).