Heard a rumor once that it was because Obi-Wan and Yoda were prepared to die, or died willingly, but Qui-Gon and Anakin died more suddenly.
This is one of the most easily answered by one word: "Cut!" Were Qui-Gon and Anaken gone when the scene cut away? Yes Qui-Gon was dying with a wound that was clearly mortal but Dead??? Did Luke truly cremate his frather or just his re-assembled metalic remains?
I have a fealing that details like this will have Star Wars nerds tearing at their pocket protectors and submitting storylines using the mission editor in Jedi Knight II! I really can't see how Lucas can tie evething up in just one 2.5 hour movie.