Another Custom in Davy Jones' Locker
I can tell by that blank look on your face that you're confused, so let me explain. In the Navy, the officer ranks are broken up into three groups: junior, senior, and flag officers. Flag officers are those that are entitled to their own flag, the admirals. Senior officers are the ones that traditionally commanded ships-of-the-line, commanders and captains. And all the officers from Lieutenant Commander down to Midshipman are junior officers. And yes, Midshipmen are officers, though they have no rank, inherent authority or seniority.
All the admirals are addressed as "admiral", captains as "captain" and commanders as "commander", which makes sense. But for the first half of my career or so, and for the previous two centuries of the Navy's existence, junior officers, regardless of rank, were addressed as "mister." You see, lubbers often get confused by their experience with the Army, Air Force and Marines into believing that a Lt. Commander is a Commander's assistant, in the manner of Lt. Colonels.
Alas, it is not so. Lt. Commander as a rank came into existence as Lieutenant Commandant, or a lieutenant that had command. Thus, a Lt. Commander is still a junior officer, and a group of them might more properly be described as Lieutenants Commander. I even have a copy of one of those professional books from the mid-eighties that cautions Sailors against addressing any LCDR as "commander" - except the XO, who is also second-in-command.
Oh well. Just one more thing I'll have to adjust to.