Because hands and fingers vary in shape, size, flexibility, range of motion, strength, disability, etc. many people have no choice but to use alternative fingerings for playing chords. When you play barre chords, you use that same A minor shape up and down the neck using your middle, ring and pinky fingers. I use a lot of strange fingerings for playing difficult chords. In most cases, it doesn't matter what fingering you use to fret a chord, so long as you can play it cleanly.