Patches, lol. The whole game is a mess of elements from different original games. There´s no consistency at all, you can use skills from other games, etc, etc. The whole point of the ending of the Aincrad-arc was that the code of SAO became public so that everyone could program their own world. Basically, our heroes are playing the most broken, unbalanced game in existence. A game nobody would actually play cause its so meaningless when there are no clear rules.