If I may. A real history and fictional lesson is in order.
This is just a commonwealth misconception about how ranks from wartime US Navy actually work. They are not the same obviously.
https://en.wikipedia.org/wiki/Midshipman
This is where the confusion starts to come into play. And why an (Acting) Ensign is not a Midshipman.
https://en.wikipedia.org/wiki/Ensign_(rank)
Now to clarify how the TSN works, looks at Star Trek please.
http://en.memory-alpha.wikia.com/wiki/Starfleet_ranks
And all of that is based on this:
http://www.defense.gov/about/insignias/officers.aspx
By the by, the title of Acting, came about from the (in)famous Wesley Crusher on TNG. Sadly it wasn’t really used since then on Trek. Odd. But it is a real thing.
https://en.wikipedia.org/wiki/Acting_(rank)