In short - there are three extra pokemon, probably legendaries, in the game files. Whether they're event legendaries or actually in the game and nobody has found them yet remains to be seen.
Serebii supposedly leaked them just to remove them. One of their names (Diancie) corresponded to one of the really old leaks.
People messed around with the GTS, using the leaked names, and the general conclusion was that they are indeed in the game's files. For example, somebody put up a Togetic asking for Diancie and it appeared available for others to see - made up names never do. Also the way the search engine reacts to name inputs on the user's end is different when using these three names compared to fake names (i.e. they load differently)
Their types were also supposedly leaked, but unlike the names which the game responds to, the types cannot be reliably confirmed. Conjecture is that they are this game's trio, and linked to the three closed power plants.
