All tanks have limited and fixed range, so Automaton just instantly splits all zerglings from the one which comes first into tank fire range. It IS instant, but highly deterministic, at least their first attacks are. And after their first shot, due to long cooldown, all zerglings are just already attacking tanks in fairly split manner and mostly unkillable by tanks anymore.
Or maybe it is because opponent is also AI and automaton simply knows which target it will auto pick.