I know a similar question has been asked already, but I was looking at another site, and it said greninja's base stat total was 530, having 95 special attack. I'm not saying that 520 is wrong, but I just wanted to ask where did you get 520 from?
If you look it up in this pages pokedex you'll see it says it is still just an estimate. I'm guessing they used the stat equation to figure it out, and depending on how large there sample is, and not being able to know the ivs it's completely possible to be off 10 either way still isn't too bad. Also if it really is 10 lower than the other 2 I would again guess that is because it is the fastest starter in xy (except maybe the blaziken with speed boost) It obviously has supereffective water attacks against the fire types also it can probably learn icebeam which is supereffective its grass weakness allowing it to sweep the other starters fairly easily if it isn't weakened a little bit.