Alternatively, he was an Ace during Episode IV and V, and then went through a narrative process to re-build into a Sentinel before episode VI.
He does very little Ace-y things in Episode VI, except piloting the speeder bike, which could be that he used some of his XP to take the Racer spec.
May not be 100% RAW, but there's a definite change in Luke's motivations and allegiance in between those two movies. He's always part of the Rebellion, but in IV and V he's an active part, whereas in VI he's still a Rebel but it's not as primary to him as being a Jedi.
Edited by Absol197