Confusion on How Courier Race Works

So I've seen a lot of Skyrim playthroughs in which people have unique couriers (not modded, just unique in the sense that they are different species and sexes), and I've even seen people say that they have Bosmer couriers in the vanilla game. Every source I see says that the appearance of the courier is not fixed, but it seems like every single time I play the game on a different save, I always have the same male Breton courier that looks like the default Breton male avatar.

I've really tried searching for answers, but I've been unable to actually find anything about it. I initially suspected that it could be based on what species you the player chose to be, because I always play an Argonian (both male and female) and it's still the same Breton courier every time, but Google AI is telling me that race has nothing to do with it.

So... DOES player race have something to do with it, or am I just absurdly unlucky?