Hey man... I'm not sure where you are getting this from but gunners are practically 100% accurate.
In my opinion, the position where you placed that archer on the hill would hinder the unit from aiming and shooting.
Also I tried the archer vs gunner test, sometimes the archers win... others the gunner win. However from experience, my opinion is that the archers do more damage when they can actually hit the target but gunners always hit and give a more steady amount of damage.