For quite some time that I am aware of this problem, and so far never found anywhere an explanation for it being this way. The option to choose what nickname to use in the user profile does not prevent that usernames still can be seen.
Of all the discussions I found about this topic, which mostly are now closed, there isn't any satisfactory explanation.
The only thing I have concluded is that there are some difficulty in understanding, explaining and find a adequate solution by many users who are aware of this issue. Which in my opinion is not a mere taste or style but a reduction of security and privacy.
I believe if I tell you, that in your website, although you hide the users, I can tell that you have two users, their usernames (one has 4 letters and the other has 3) and which one is the administrator, probably you do not stay very satisfied do you?
This question should not be undervalued, i'd like to find a better solution. Maybe a detailed official publication with some clarification with a roadmap? At least could be identified as a lack with a warning in the main documentation and the best options available.
Unfortunately I do not know a simple solution that I can suggest.
But I leave here this example with the Matt Website http://ma.tt/
You can find at
http://ma.tt/?author=1 the admin probably is saxmatt as you are redirected to