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Think about it.
Pikachu starts off looking weak in every new region, but the damn thing hardly gets trained. Not like they used to train, that is. Not like they trained when Ash and Pikachu were forging their bond. This same Pikachu then somehow more than keeps up come league battle time. He's holding the little shit back on purpose. If he went full on with new trainers in every region, Pikachu would kill their pokemon. They both have to play softball.
I mean it's an illegally supercharged pikachu to boot. Ash's ass would hang if he didn't keep it covered up.