It must be some sort of bug in the level scaling system. I've never gotten anything like that at low levels because I doubt it's supposed to happen.
Just keep it, you're lucky you actually got a weapon that's able to kill enemies easily.
I found an elven bow at around the same level once,another great find for the level.
I don't think it's a bug or anything,it's just a rare accurance that sometimes spawns.
I would also keep it,heres why.
Weapon skills go up by how much damage you do,without enchantments or poisons,that is the total damage of the weapon its self.
This means fewer shots with a stronger bow will increase your archery skill faster or just as fast as more hits with a lesser bow.
Increasing your weapon skill is most likely your primary objective,at least for fighting enemies.