There was some discussion on BoardGameGeek about the campaign ending, and I agree it doesn't make much sense :

Losing the mission before the last one, and succeeding in the last mission results in a rebel victory, while succeeding in the mission before the last results in a much harder mission(thanks to Vader, according to the BGG consensus), and an overall defeat if the last mission is a failure.

It would have made much more sense to have something like :

n-1 : victory, n : victory : Rebel major victory

n-1 : victory, n : defeat : Rebel minor victory, or draw

n-1 : defeat, n : victory : Imperial minor victory or draw

n-1 : defeat, n-1 : defeat : Imperial major victory

This would be more consistent with the outcome, and give no reason to try to botch the mission before the last on purpose.