Perhaps, opposing players haven't high skills, but maybe they've very good secondary skills like off the ball or balance. The height of the players also have a role in the game's engine. That is, a tall player would have more chances supposedly to heading the ball to one of lower stature. On the game's manual says:
Beat the defenders to win a heading:
Action: Jumping, Heading, Off the ball, Strength, Balance, Technique
Player rival: Jumping, Marking, Strength, Balance, Heading, Positioning
Excuse my english, google translate don't help so much
