FFXIV was designed for ease-of-understanding even if you've never played a Final Fantasy game before.
The number system actually predates the -a, -aga, -aja, system as well in the really old FF games.

If you want to use the opposite name convention, macros can solve that for you. Just create a macro that has

/micon "Fire II"
/ac "Fire II"

Name your macro "Fira" and move it to your hotbar.