Why is Ash's Pikachu able to KO Brock's rock/GROUND type Onix with an ELECTRIC type thundershock in the early Anime episodes? probably because the Anime is 1. geared towards kids who aren't gonna overanalyze it and 2. a cartoon based on a Turn-Based RPG, which wouldn't be very entertaining if the battles happened as they do in the game.....
ever seen Pokemon in Real Life? look it up on YouTube. that'll answer your question :)