Short answer : it's not a bug.

Long answer : there's a group AI going on. If your archers are in a specific group, and you select them through the group to order them to fire (ie either click on the group tab, or select the group through keyboard shortcuts), they'll split their fire over the enemy battleline as best they can, selecting best targets on their own. Same group code going on than when you make an infantry group and order it to attack the enemy battleline : the AI tries to ease out your workload.

Of course, sometimes this AI prerogative can be a bother. If you really want all your archers to target one single unit, either ctrl+double click select them or ctrl+single click 'em all, and then right click order them to fire. That does the trick, and tells the group AI to back off.