I never said Garchomp was bad. I just think some Pokemon, like Torterra, Infernape, or Gyarados, might be better, and Yorec's comment did not address this possibility. All 3 of these Pokemon learn powerful moves early in the game, have trouble against only one gym, and can handle most of the Pokemon League by themselves.
There are several reasons why a Gabite might reach level 48 after defeating Volkner. Some people like using lots of Pokemon and consequently giving less experience to each one, and other people like going through routes without battling all the trainers. Last time I played Platinum and recorded every Pokemon's levels, my Pokemon earned about 516051 experience before Volkner. This is enough to evolve Garchomp if I used 4 Pokemon but not if I used 5.