You may be stronger because your tendon attachment gives you better leverage, you may have more fast twitch fiber, or your nervous system may be able to activate more of your fibers at once. The only thing you can do is train for mass, and not strength. This means don't do sets of less than 5 reps. Instead hit all of your fibers by doing sets of 5-12 reps. Keep experimenting since you may respond better to one type of training.