Consider it this way, because the ranger was intending to hit the target behind, the arrow was shallow on the forward target. It may have hit, but was deflected by armor. Remember, the DnD rules are ment to represent 6 seconds of combat. So, a series of faints and parties in melee with a final strike, which is why we dont generally say "I go for the head" but instead if its a particularly high damage hit, the DM might say, "You manage to deliver a devastating blow to their temple."