I have very extremely busy fingers, from typing on keyboard, or hitting buttons on calculator. Sometimes, my fingers and my brain don't agree with each others. I want to close one tab, the leftmost one, but end up close the 2nd tab from the left.... And this does happen often, sadly.
Undo Closed Tab works by memorizing position and session when the last tab closed, be it legitimate or accidental closing. This can be done when multiple tabs closed, like Windows common undo and redo functions. There's no need to reconfigure after installation, the addon should configure itself and work correctly on most conditions.
Now that I've posted this, don't whine here if you got your tab unwittingly closed by your own fingers. Don't say I didn't warn ye! :P