Perhaps I'm oversimplifying this, but maybe they just picked one direction and universalized it. Maybe it wasn't a choice of superiority, but just a capricious decision made by a famous map maker, and it caught on, just like how misused words catch on.

And in response to: New puzzle - why would the wonderfully whimsical spell-checker suggest "O'Brien" as a replacement for "NZ"?

I would say that it's because this spell checker goes to the next alphabetical word, and seeing as there are no possible words beginning with N that can come after NZ, it moves on to the first existing word that begins with O, this being O'Brien because numbers, apostrophes and other non-alphabetical characters come first.