Let there be a character. Let the character's name be, for example, Alexander.
Now, Alexander's parents call him 'Sasha'. His friends call him 'Xander'. His girlfriend calls him 'Alex'. In formal circumstances, he's 'Alexander, son of Philipp'. To himself, he is all of those - they are, after all, variations on a theme.
What should the omniscient narrator call him? That is, if the omniscient narrator sits on the parents' shoulder, Alexander is called 'Sasha'. If the omniscient narrator sits on the girlfriend's shoulder, Alexander is called 'Alex'. But what if the narrator is sitting on Alexander's shoulder, or on no shoulder at all?