Weapons have ammo. If the ammo is "default", the damage is what the weapon is supposed to make. If the ammo is better, there is a positive modifier to the damage. If ammo sucks, the modifier is negative - the bow still does damage, but less then what it does with "regular" ammo.
Just arrows are well made - not bent, no dents... So, the modifier is positive. + 1 modifier.
Barbed arrows are even better. + 2 modifier
Bent barbed arrows are worse because they are bent, but that is somewhat bakanced by them being barbed. 0 modifier - no modifier exists and, therefore, none is shown!
Bent arrows suck, so the modifier is - 1.
Since all bows do more than 1 damage, there is always some damage done by ANY arrow.