Not all authors appear when adding or editing article in the backend. Only those with backend access.From other perspective it is very annoying when admin edits an article submitted in the frontend, because the article author is changed automatically with the user who edits it.

My fix when editing someone's article in the backend is to quickly make them administrator status, edit their article, choose their username from the author drop-down, and save the K2 item. Then go back and switch them from Administrator to Registered.