Honesty forces me to tell you that I'm not sure. My guess is that Austen didn't want to name names within the context of the military to avoid problems. After all, Pride and Prejudice takes place in an era during which there was great worry that Napoleon might cross the Channel!
Who knows, perhaps authors weren't even allowed to do that, even if the names were fictional.
I promise though, to check this out more thoroughly.
You'll also see that with Austen & her contemporaries, they use a similar thing (for lack of a better term!) to denote a titled individual, such as Lord ______ or Viscount _______ . This was done so as not to offend the individual who held that title, as it was assumed that you were writing about *that* person.