It is historical. Originally, we had one captain and the rest commanders. Now we have one fleet captain and a captain with others that are being considered for the next captain rank. There is a difference between the rank of Captain and the position of ship captain though. Ship captains can officially be as low as Lieutenant Commander, and I am talking on a permanent basis, not just stepping in when the captain is absent. It is more likely that ship captains hold the rank of Commander and above though. In our division, there has only ever been one ship captain that held the rank of Lieutenant Commander.