Why does my course listserv list show many more subscribers than the number of registered students in my course?

For any given course-related list, each registered student is subscribed to the list by all three of his/her standard email aliases, and each instructor is subscribed to the list by all of his/her email aliases, including any personal aliases he/she may have created in EDS. E.g. if a course has 1 assigned instructor and 98 registered students, the associated list should have approximately 291 subscribers.