Comment by TRiG_Ireland

Since the list was originally developed for post, there's precedent for far-flung regions of a country to have their own codes, even when they're not considered politically separate. French Guiana, for example, is considered fully part of France, and yet it has its own code (GF).

The Chagos Archipelago is a good distance from the rest of Mauritius, and so it may perhaps retain a separate registration on the ISO list.