In the Star Wars universe, more like the real world, there are many factors during a duel. The Windu vs. Sidious duel, according to George Lucas, Sidious was in control for the whole time to lure Anakin to the dark side. He may not be as strong as Windu in terms of lightsaber dueling, but again there are other factors including manipulate other people and etc, which Sidious was the grant master of them.