I thought ESB and ROTJ were still the “4.2” six-track?
Maybe someone can chime in and clarify this.
Apocalypse Now was the first movie to be mixed in 5.1 when it was released in August of ‘79, less than a year before Empire’s release. It’s possible they kept mixing Empire and Jedi in the older 4.2 configuration since the 5.1 format hadn’t become the standard yet.