Mate straight up, you don't get it.
I've racked up near 1000 games online between fifa 13 and fifa 12. The bottom line is that the ai on your team continually make crap adjustments when contesting a header, too many times they leave guys wide open or simply run from the ball. Furthermore for some bs reason a bloke with amazing height, jumping and heading abilities gets outplayed by attackers that shouldn't even get close.
Notice how nearly every game involves crossed headers.
Notice how people run down the wing, cut inside and run to near post to draw the defenders to cross to a team mate wide open? It's cause the ai do not adjust. They play stupid.
I'm not saying that you should never try a header goal, but simply stating that players that simply use this tactic every time they have possession are simply looking to exploit the ai.
Ps - both geo and pennywisealfi are two of the most fair players I have seen.
Select the player then select make offer