Heading away unchallenged comes under clearances, of which he has made 121 in 16 games.
A defender being in contact with Aguero is them winning a duel..If Aguero is stood away then it's just a clearance.
A keeper can save a penalty, it bounces off the post hits them and goes in..They still saved that penalty .. Otamendi gets his head to the ball above his opponent, he's still won that duel. Maybe if he was about 7ft he could direct header to where he wanted every time, but there is body to body contact you can't win every one cleanly(he's not coming up against Aguero every week) just like you don't know where the ball is going to end up after a 50/50 tackle.