When I say that I mean when Iris has a level 50 something hydreigon when it doesnt evolve until level 64?
Well obviously GameFreak wants to make the Trainers/Elite4 somewhat challanging to an extent, but not to an overpowering level. This means they have to put in Pokemon with high base stats - but not at the levels in which they're supposed to evolve. It has happened in basically every Pokemon game =)
They do this to increase the stats of an opposing NPC's Pokemon, to make the game a little more challenging. It's possible to get over-leveled Pokemon in the wild, never under-leveled evolved Pokemon though.
Hope I helped. :)