Luke didn't slice off Vader's hand in SotME, he only mangled his arm.
Y'see, Vader unleashed a kinetite attack. A kinetite is kinda like the Star Wars version of a DBZ move; it's a globe of telekinetic energy that carries enough concussive force to break down an armoured ferrocrete wall. Luke deflected the kinetite back at Vader, which crushed his side (i.e., broke bones, probably with massive internal injuries). Vader then tumbled down one of the ubiquitous Bottomless Pits<SUP>TM</SUP> that Jedi love to fight around.
By that point (presumeably), Vader had already been called on the carpet over losing Death Star (as if it was his fault
http://www.jediknight.net/mboard/rolleyes.gif) ), and had added an artificial hand to his already impressive list of cybernetics. This is what Mara referred to.
Actually, the real reasoning was that showing a bloody stump would've earned RotJ a PG-13 rating (at least). Note that, when Luke lost his hand, the scene was blurred and that Luke quickly buried his stump in the crook of his other arm.
So the score is:
-Luke destroys DS. ===> Vader loses right arm for "his" incompetence.
-Luke injures Vader on Mimban by deflecting his own attack back at him.
-Vader lops off Luke's right hand on Cloud City.
-Luke chops off Vader's (cybernetic) right arm at Endor.
That was when Luke realized that he was becoming as vicious as his father (watch how he gazes at his own artificial hand), and proceeds to throw away his lightsaber. <small>. . .right toward the shaft. Oops.</small>
http://www.jediknight.net/mboard/rolleyes.gif)
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"The Beasts know much that we do not." -Ancient Jedi proverb
[This message has been edited by BeastMaster (edited March 24, 2001).]