It's the same thing Lucas is doing with all the Star Wars.
Releasing them all individually on VHS/DVD. Then he'll release them as a trilogy set (1-3 AND 4-6). Then he'll release them as a whole movie box set (1-6). Then he'll 'Digitally' master something else and start all over again. There for making 10 times the amount of money off of 1 movie because everyone wants to have the new and improved one and wants to have the official trilogy and set of them.
