In reply to a question by Jan Boddez in reply to my earlier note.

No I don’t use a plugin or filter to change them. Comparing it to another WP site, the difference seems to be using dots or spaces in user names. Those break the author urls that would result from using the actual login user names, and likely WP tests for it?

That’s interesting. You must be using a plugin (or filter), then, to change author slugs?

Jan Boddez