A person in the system could have several different roles. A particular person could have a student, staff and faculty role. When assigning roles to a person, you should assign them the highest role.
Nevertheless, sometimes a mistake is made when a student with only the student role is assigned the wrong person type. In order to find students with wrong person type, use the following query.
SELECT a.stuID, b.fname, b.lname, b.personType FROM student a LEFT JOIN person b ON a.stuID = b.personID WHERE personType != 'STU';