I've only seen the highlights but what struck me was the ball was headed on the six yard line (or possibly a little inside of it) and more or less directly in front of Arnolds starting position. Whilst I realize the scorer was in front of him, could he not still have come and punched/claimed it?
Just watched the goal and you're right. He takes about three steps away from Arnold to the six yard line and has an unchallenged header. If we had man marked and Arnold commands his six yard box, that wouldn't have been the so easy.