Even if you compare shots of ROTS to shots from ROTJ, they actually don't look that much alike. They do, but they don't. In ROTJ it is very easy to believe that he is just a very old man, nearly a walking corpse. In ROTS it just looks like rubber and makeup, and he has more of a monster or alien look to him than the ancient withered look of ROTJ. He was already a fairly old man, 25 years turning him inot the walking corpse from ROTJ would have been reasonable. The way it goes down in ROTS just feels forced.

So yeah, I would have to say it was just another balls up by George.