There is no such thing as total balance. Even in a game like Halo where every person is Master Chief there still isn't balance (whoever spawns near the best weapons has an edge over the others).
Some champions have their numbers/abilities slightly over tweaked. RIOT does this on purpose (pretty sure they admitted that...too lazy to find source)
It adds spice to the game and gets players to figure out the power of the lastest patch...and also figure out how to counter it. If it gets too rediculous there are always bans...
This is a game of synergy, counters, and cupcakes....not balance.