sephiroth in AC made some sense. kadaj and the others didnt really. they were remnats who seemed to have appeared out of nowhere. none the less they are seen as part of sephy. so mix some jenova cells with a party of him and jenovas ability to change shapes brings back sephiroth. maybe it wasnt the real sephiroth but it was jenova and a remnant taking the shape of sephiroth and haveing the mind and power of sephiroth.

so idk you can try and make sense of it.