I am learning FERAL ROOTS by Rival Sons. It is in Drop D tuning. He also uses a Capo at the 3rd fret. It says it is in Fm. I am a bit confused as to what exactly is happening here with respect to music theory, key, etc. I believe his notation (and noted key) does NOT factor in the capo being at the 3rd fret (common practice in tab) yet if I play over the main riff at Fm (penta pattern) starting at the 13th fret, it works great. I ignore playing the top E string because it doesn't work since it is tuned down a whole step to D. There are only a small handful of notes in the tune so it doesn't really even fit a key perfectly- something I have leaned lately. Key is just a convenient word- for many tunes, it is much more about the actual notes used and chord changes. By contrast in Blues- key means a lot and I can use all the notes.

So my question is, would it still be in Fm if the capo is at the 3rd fret? Probably not but then why does it work at 13th fret. Am I in Fm but changing a voicing? If I go backwards from 13, the 3rd fret is closer to the 2nd position in the pentatonic scale pattern- not the first position.

What is actually happening with the addition of the drop D tuning and use of the Capo at the 3rd fret?