On Kawaii's note, I, myself have had the same complaint... Coming from... Myself... I just think it's a bit redundant to name each one Final Fantasy... But alas... I came to a conclusion that should satisfy my retentive nature...

I look at it in the way that each, "Final Fantasy", is the "Final Fantasy" for that particular setting... It's like, Square developed multiple dimensions and universes, (all of them possessing a Cid strangely), and set a "Final Fantasy" in each... So while they call them Final Fantasy 1-12 and such, the number referrs to the corresponding universe/dimension... It's a vague explanation, but it's all my tiny little brain can think of...