I have watched all the shows in the Black and White seasons and remembering all of the moves, he had: Thunderbolt, Quick Attack, Electro Ball, Iron Tail, Volt Tackle. How is this possible???
I'm pretty sure Ash's Pikachu forgot Volt Tackle for Electro Ball, as to my knowledge it hasn't used Volt Tackle, since it was first shown using Electro Ball.
Bulbapedia seems to confirm this, as it states that Ash's Pikachu current moves are:
Well, it's the Anime, not the game so who knows? If the anime does correspond with the rules of the game, Ash most likely replaced a move.
Ash's Pikachu forgot Volt Tackle because Pikachu wasn't used it since he learned Electro Ball.