When an ostrich puts his head in the sand he can't see you, but you can see him. Same principle. I kill and get killed like this quite often. Oddly, even some very experienced players think that when their commander and gunner have no vis of the enemy, it means the rest of their tank is hidden.
Add to that trajectory issues where one player can sneak a shot under or over a tree while the other is in a spot without a clear path. The path of the round is different than the straight line of sight through the sight. Often you can't see a tank, but your round coming in from above has a clear path. Just like killing guys that back down out of sight on a ridge.