Originally Posted by Fat cantab
To be honest, the real question is why do all non-traded Pokemon get the SAME experience. (With Natures in RSE, they kind of don't.)
Perhaps it has to do with trainer interaction. Since the non-traded pokemon would be used to taking orders from trainers, it would be easier for them to win fights. This would result in them getting less experience points. Conversely, traded pokemon would have less experience taking orders from their current trainers; and as of such, would have to work harder in order to win a battle. This would result in attaining more experience.