to explain a bit more. In the web interface a user can see another user information when he goes on the:
- user profile page (http://mymoodle/user/profile.php?id=USERID) => core_user_get_users_by_id().
- course user profile page (http://mymoodle/user/view.php?id=USERID&course=COURSEID) => core_user_get_course_user_profiles().
These two pages do not display the same information in Moodle, so it does not with the web service functions. If ever the web service function does not display the same things as the web version it's an issue. Otherwise it works like excepted. I hope it clarify.
Note that except if you are admin, you are most likely never going to see someone else username in both pages.