Tiers are based off of usage on Pokemon Showdown. Which is a Simulator. Which means you can get a flawless Keldeo like that. In game however, people tend to RNG legendarys instead.(or in the case of X/Y, Soft Reset)I notice a lot of Legends fall into the OU tier, among others, but my question is why/how are they so prominent given that you cannot breed them. Are there so many hacked perfect IVs out there that they are commonplace and easy to obtain if desired? I want to make an OU team but not with any legendaries in it, and it seems most have at least one.
While this doesn't give you a full answer, it does explain why there are many legendarys in OU ^^
EDIT: Ninja'd