Both answers above are great....I used to use a guy who was a nib grinder( yes it's a thing) and he explained it to me...its basically as Duncan has said, it's pen presentation, how it touches the paper,
Some of us lefties overhand write, some underhand, some drag and some twist the paper and write up the page...that's why some nibs will suit other better...
Also if you have a label cast blanks or engraved barrel it needs to be the other way for south paws....