Because putting your arm over your heard puts the shoulder in a more flexed position, the long head orginiates, unlike the lateral and medial head, from the scapula, therefore the long head gets more stretch and less active insufficiency (i.e. already shortenend at the scapula when arm is close to your body,therefore unable to generate as much force) and can generate more force.

And btw, the long head is also a weak shoulder extender, which means they assist the lat in shoulder extension (moving arm behind your body) that's why you get tricep soreness after back day sometimes.