That is probly due to how it is scripted, and a good thing as well since poisoning individual arrows or bolts would be rather costly and time consuming since you would have to poison each missile with a individual potion. The end result is the same, poison the bow, and it lasts for so many arrows. If you took a potion and soaked a number of arrows they would last about as long as the poison on the bow lasts. Though I admit it is odd to poison the bows.
AHHHH it just came to me, Drocket scripted a small vail that fits inside the bows near the buisness end. You are not "poisoning" the bows so much as filling the vial so that the missile rubs on the poison soaked pad similar to a bingo marker as it is fired. Once in a while ya gotta refill the applicator. See it isn't so strange after all. lol