I assume you have contacts with First Name, Last Name and contacts with either a first name only or only a last name - or some with just email addresses. The sort feature works this way: Ascending: Last Name, First Name; Firstname; Screename. Descending works the other way around.
So assume you have two contacts with "Joe Alpha" and "Jim Delta", once contact only named "Bravo" and one with email address only (charlie@aim.com), the sorting would look like this: "Alpha, Joe"; "Delta, Jim"; "Bravo"; "charlie@aim.com"