As the first Crispin's links shows, index hint is deprecated and it's ignored, so even if it used to be related to the question, it's not relevant anymore. (The link it broken at the moment; I'm going to fix it.)
I think the most likely explanation is that the additional field works as an included column (regardless if it's actually set that way or not). If there is a query that needs just these two fields and the database can take them from the index, it doesn't have to make a lookup to the table at all, which saves resources.