why some users do not appear in mdl_user_info_data in Moodle?

Question

I am getting students added to course using a query but i noticed 2 of them do not appear( even if they are active and listed as participants) checking inside tables i noticed they do not appear in mdl_user_info_data. How can i prevent this? or whats the reason they dont were added to this table

This is my query:

SELECT u.id,u.username,u.firstname,u.lastname,u.email, b.data
        FROM mdl_user u,mdl_role_assignments r, mdl_user_info_data b
        WHERE u.id=r.userid
        AND u.id=b.userid
        AND r.roleid=5
        AND r.contextid = 'somecontextId'
        ORDER BY u.email ASC;