Pokemon Levelling up quicker??

[Karlosnite]

I have my party of 6.

2 are holding mega stones.
The other 4 are holding element plates.

I've noticed for a while that my Luxray is levelling up at a high rate. Gaining between 3000 - 5000 xp depending on the battle wheras my others only gain 1/2 xp of that.

Why would this be?

I caught Luxray as a Shinx

 

[Altairis]

I have my party of 6.

2 are holding mega stones.
The other 4 are holding element plates.

I've noticed for a while that my Luxray is levelling up at a high rate. Gaining between 3000 - 5000 xp depending on the battle wheras my others only gain 1/2 xp of that.

Why would this be?

I caught Luxray as a Shinx

Some Pokemon gain EXP at a different rate than others. I don't really know why this is the case, but it's true. Did you trade this Shinx from another game? Pokemon gain extra EXP if they're from another trainer, holding a Lucky Egg, or if you have the EXP O-Power in XY or ORAS.

You can see these rates of growth on Serebii's Pokedex pages. They're listed right below the abilities on the left. Luxray's growth is "Medium Slow", you can compare this to the other Pokemon you have on your team.