Why do I level up so quickly if I fail?

I've recently started playing League of Legends, and lets face it, I'm not the best. However I do find that I am regularly hitting level 18 at roughly the same time as players with much better scores. Why is this, and why am I not penalized for my failings?

Solution 1:

There are a number of reasons this could be happening

1. You farm minions well instead of champions
2. You are getting the assist kills
3. You have the runes or masteries that boost your XP gains
4. The XP scales up over time so you will level faster the more time goes on
5. You get global XP for destroyed turrets

I am sure there could be more reasons, and also remember that 18 is the cap.. who is to say they would not be 20+ when you hit 18 if it didnt cap.