I was told in-game that "you don't get a damage bonus past 5 PT on the expensive weapons (jarids, lance, javelins, etc.) but that there is no limit with the lower weapons like rocks and throwing knives."
I have 9 PT this generation, and have one-hit people with everything but stones. Throwing knives, heavy throwing axes, javelins. I've also turned people (or their shields) into pin cushions without them dieing as well. Sometimes I will get 0 to 1 point (meaning I did very low damage?) with throwing spears and jarids against horses.
Perhaps there is a randomness to the damage equation. Maybe the velocity of the weapon has a big effect on its damage. Who knows? Not me. I just throw things at people until they die.