My head cannon is that it’s more of a positional title than a rank. The real US Navy does this all the time. You can be an O-5 (rank of Commander) and be called a Captain if you’re the CO of a ship. Similarly you can be called “Commodore” if you run a squadron of boats which is usually a rank of O-6 (Captain).

So my guess is, in the time between ANH and Hoth in ESB, Han led a land campaign and keeps the honorific. Luke is sticking to his X-wing in that time and (due to his growing Jedi-ness) is apt to be a supporting team player in an existing squadron.