I believe the explanation of that is:
George Lucas: “Oh shit, we only have thirty minutes left of screen time to get this done… how about Mace Windu blocks his lightning to make him look like that?”
George’s poor employees: "Yes, perfect, all of your ideas are amazing George please don’t fire me"
Yeah forget the fact that Empire is almost two decades later and there’s plenty of time for him to get ugly between the end of Sith and the time when the Emperor contacts Vader in the asteroid field in Empire.