You need to give it a few hours to get a better reading. If you are using your phone then it is normal for android system to appear at the top of the list. However if the phone is idle then you would expect to see cell standby or phone idle above it.
Android system encompasses lots of different processes which other apps may require use of. For instance if I spend time playing a game such as smiley pops that will increase the battery stats for android system, presumably because the game is using screen, sound, vibration, etc.