For whatever reason, at some point in the past, the Space Forces and the Confederation Navy were different organizations. Perhaps the Space Forces are just the Spiritual Successor of some Air Force or another (or all of them, since this is all of Earth's militaries combined). Once they were merged, units that originally had been an Air Force/Space Force unit maintained that identity and the associated rank structure, and Navy units similarly did the same. So if you found yourself aboard a carrier that had a Navy wing embarked, you might be Lieutenant AFGNCAAP, while if you were instead assigned to a carrier with a Space Forces unit, you'd be Captain AFGNCAAP.