>Shinx/Luxio/Luxray have a maximum of 1,059,860 Experience Points (Medium Slow).
>This is the same for Starly/Staravia/Staraptor, having 1,059,860 Experience Points (Medium Slow).
What this means? -
When your Luxray is at Level 70, it will have a 274400 of total experience, and Staraptor, beeing in the same ''Experience Group'', at Level 70, it will have 274400 to.
Also there is one thing that maybe you didn't know, and maybe it's your weird problem -
When you have a Pokemon with a different ID from your game, it will win a free boost on the experience.
What did I say? -
If your Luxray was captured in your game (having the same ID as your game) and your Staraptor MAYBE be traded from another game, Staraptor will win more Experience Points than Luxray.
There's also another thing, MAYBE some of your Pokemons were holding a Exp. Share/Lucky Egg and you don't know.
Also check THIS , it's were I see that Luxray at Level 70 have 274400.. That maybe be useful for you.
Hope this helps!