That Pikachu has the Light Ball... that is the ONLY explanation (besides Plot Device) that Team Rocket has for chasing the 'supposedly rare' Pikachu that Ash has. Therefore, I can pretty much say that (assuming Pikachu was level 20-25 two months after Ash started with him) Pikachu's massive growth in power is recessing and the rat is actually starting to get weaker. It's become less a Pokemon and more like a human. So it's basically personality vs. power. And currently, Pikachu's personality is on the way up, while its power is on the way down.
Because, let's face it... there's NO way a Pelipper should've taken a thunderbolt, grounded or not. Trees ground lightning bolts... doesn't mean they don't split in half while doing it.