at this point, I have no idea. 
I was told that it takes so long to send email to all of the students in a MOOC, and email is often sent when a course first opens and additional enrollments are coming in, that in a direct comparison the "total" for an email message and the "total" for a course will not necessarily match. 
@brianhw, could you please tell me what the **Total number of enrollees** shown on the **Course Info** page is tallying, and how frequently that number is updated? 
I will then try to understand how the "total" for a bulk email message might be different, and how best to convey that difference.