Because it's been mentioned several times throughout the EU, and alluded to in the movies. A jedi at his greatest possible potential is stronger than a Sith could ever be, but otherwise most sith would be stronger than a jedi with comparative training.
Like this:
The Sith start out stronger and stay that way for a long period of time, but eventually they peak whereas the jedi continue to grow.
And I should also add that being more powerful =/= the one who can hit hardest with a lightsaber, that alone isn't power, and Vader didn't win the battle in episode IV, Obi-Wan did he stated "If you strike me down, I will become more powerful than you could ever imagine" and he did. In death Obi-Wan became far more powerful than Vader, and I'll say it again, that has very little to do with his skill with a lightsaber. Dead man obviously can't participate in lightsaber duels.