Katanas, and a few other melee weapons have more base damage. Fists, knives and fire axes have a base damage of 2, katanas, hammers and gardening tools have more. The way I understand it, maximum damage is something like
base damage * ((feat + lethality) - skill requirement)
I think it's like this, but I don't know for sure. Knives and fists are faster though, and that's more useful againts some nasties imo.