The answer is found in all three mediums combined : TCG, Videogame and Anime. The TCG deals with energy management and chaos management, the videogame deals with strategy and stat management, and the anime deals with real life style improvisation and bonding.
First,From the TCG perspective, Pickachu's attacks would be defined as being powerful as the number of energy cards are attached to it. In anime terms and videogame terms, it's how long did pikachu have to charge. According to the TCG, higher level mons take longer to charge and attack, thus allowing quick weaklings or speedy glass cannons to knock them out. This is definitely seen in the anime.
Second, from the anime, since the anime has real-life-like settings, just as in real life if you don't do something for awhile, lose concentration or confidence, you will lose some efficiency. In a similar way, Pikachu's potential is great, but whether he or Ash is able to use it is another story.
Third, the anime mentions aura (chi?) as a source of power,etc. So there is some in-story mechanics that we don't fully understand.
But in a story where little penguins can summon tidal waves in the middle of the desert, the oddities of a certain pikachu shouldn't be surprising nor of concern.