Many years ago, I read that masters for ROTJ couldn't be found at the time, so they used 3rd or 4th generation copies instead... hence why the quality is so poor. With ESB, it mixed incorrectly without care, but I'm not sure if the original masters were used. Also, I've read that better, higher quality masters for ROTJ were found, but the article didn't provide any more information. To my knowledge, none of this has been confirmed and the link to my source is now dead, so who knows.