>>45832728gen 5 has anti-grind leveling where you get massive exp penalties for defeating mons of a lower level than you. Since fighting every trainer on a route will more or less always put you at the level of the next gym leader/boss fight/E4, fighting wild Pokemon in Gen 5 basically won't level you up since they are always lower level than the trainers on the same route.
On the inverse you get massive exp bonuses for fighting mons at higher levels than you. So new mons you catch will near immediately rocket up to where the level curve wants them to be, and if you skipped trainers the ones on the next route will give way more experience to bring you back up to the level curve.
