They mentioned levels before, and probably knew they wouldn't be able to keep it consistent, so dropped the idea of having actual numerical levels comparable to the games at all.
Actual experience points have never even been mentioned in the anime. "Experience" seems also to be another term they use just to use vocabulary from the games, in that case, it just means whenever the writers feel that the Pokemon should evolve. Actually, they don't even all evolve in battle, like Swinub evolved after pretty much no battling while against a Team Rocket mecha. Some Pokemon hardly battle and evolve, like Starly or Swinub, some like Chimchar battle a lot and it takes a few seasons >_>. They're not gaining actual experience points, they're not going up levels, gaining EVs, or anything like that. They just battle, and the writers randomly decide whether they're actually going to get any stronger, evolve that episode, learn a new move, whatever.
Really, the writers are using words like "levels" or "experience" just to use words from the games, when what they're doing has nothing actually to do with game levels or experience >_>
The Pokemon Special manga mentions actual levels and keeps much closer to the games than the anime, but I don't think even Pokespe mentions Pokemon gaining actual experience points.