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Hey, maybe Lucas can insert some overly-expository dialog into the next edition to clear this up.
Vader: "Your powers are weak, old man. Clearly your skills have diminished since the time when we dueled on the volcanic planet of Mustafar and I made the mistake of letting you get the high ground when I otherwise showed the superior saber technique."
Obi-Wan: "Only in your mind, my very mechanical apprentice. You can't win, young Padawan, who once was called Anakin but now clearly has no good left and therefore can simply be called Darth. If you strike me down, I shall become more powerful than you can possibly imagine, as I have learned the ability to defy oblivion, as was once accomplished by a Shaman of the Whills. It is a state acquired through compassion, not greed. To commune with the force and become one with it while retaining my consciousness was taught to me by Qui-Gon since I last saw you. You will see."
Vader: "You should not have come back. My Sith training has allowed me to enhance my point of view that all Jedi are evil. By channeling the dark side through my mechanical appendages, I can cut you down faster than you can defend your position. I can tell by your slow moves that a man of your age needs the dark side to remain quick. Just like Dooku, I mean Tyrannus, proved to us on the bridge of the Invisible Hand back when we rescued Palpatine from the clutches of the cyborg General Grievous. Clearly the dark side gave him the upper hand on the Invisible Hand -- hey I made a funny. And if you will allow me to give a syllogism, Dooku, I mean Tyrannus at his most powerful, was able to render you unconscious; I was able to kill Dooku; ergo I will be able to kill you easily because the dark side has made me stronger while your fighting skills have deteriorated in your solitude.
Obi-Wan: You have been watching Young Indiana Jones, now available on DVD in stores everywhere and from your favorite online retailer from Lucasfilm, Ltd. haven't you?
Vader: "You underestimate my pow-errrrr."
Lucas: "Cut. Faster. More intense!!!"
Vader: "Youunderestimatemypowerrrrr!"
That kind of fantastic Lucas dialog will clear this up once and for all.