Why does my Greninja get boosted EXP?

Solution 1:

While evolving a Sylveon for a trade, I stumbled upon a solution. Once I was ready to level up my Eevee, I noticed that he too was receiving boosted EXP. As it turns out, I had messed around Pokémon-Amie with Greninja and he ended up with 2 hearts of enjoyment. To verify this hypothesis, I played with a spare Gabite, defeating a wild pokémon after each additional heart.

With a single heart, there was no noticeable change to the Gabite: she still earned normal EXP and didn't seem to have the thelepathic abilities Sylveon displayed. Once she earned her second heart, she immediately started earning boosted EXP. She also starting dancing at the beginning of battle as Greninja had.

Earning two hearts of Affection in Pokémon-Amie will boost EXP earned for the beloved pokémon

Solution 2:

Boosted EXP without being traded may be a result of the EXP Point O-Power, which somebody might have buffed you with. If you were connected to the Internet through the Player Search System (PSS), this is the most likely case. To get this O-Power yourself, you must talk to Mr. Bonding in the Anistar Pokemon Center.

Also, it seems that increasing your Pokemon's affection and related stats through Pokemon-Amie has a chance to increase EXP gained along with a host of other effects.

Solution 3:

There is an additional way to get boosted EXP that has yet to be mentioned (though it obviously doesn't apply to your situation here).

Aside from trade (which you already mentioned), Pokemon will get boosted EXP if you stop a level-based evolution from evolving.

My Weedle, Steve, is now level 8 and gets boosted EXP for every battle we're in. Were I to let him evolve into Kakuna (this naturally happens at 7), this boost would stop.

Solution 4:

Okay so I caught a Gible in the wastelands, and I've never battled with it (not even had it in my party) until today, and in my first battle, it gained boosted exp. I've never super trained with it, nor have I played with it on Pokemon-Amie. I have no idea what is going on with it. But it would seem that other Pokemon do get boosted exp when played with and their affection goes up.

In all honesty, this could be a glitch from the game itself. Infecting random Pokemon. Because none of my other Pokemon are exhibiting this symptom. I'm thinking about contacting Game Freak or something b/c I don't want my progress erased. You never really know..

Also, I've done PA (Amie) up to two hearts, and that Pokemon never gained boosted exp. I'm gonna try and do more research on this.