Han & Luke's Rankings

So in The Empire Strikes Back, Luke is a commander and Han is a captain, but in Return of the Jedi they are both generals, now my question is, how is this possible? I mean they couldn't have been promoted inbetween films because Luke was on Degobah training to be a jedi and Han was frozen in carbonite, neither of them were with the rebels in between movies so what did they do to get promoted? Especially to something as high a ranking as a general!