CourseInfo tab on instructor dash uses:

 enrollment_number = CourseEnrollment.objects.filter(course_id=course_id, is_active=1).count()

That's the number of users enrolled in the course.

Bulk email makes the following query:

            enrollment_qset = User.objects.filter(
                is_active=True,
                courseenrollment__course_id=course_id,
                courseenrollment__is_active=True
            )

That's the users that are enrolled in the course who have also responded to their activation email.   That's what "is_active=True" means.  The second active clause, "courseenrollment__is_active=True", looks to see that the user has not unenrolled from the course after enrolling.  

We do not want to send emails to those users who have not yet confirmed their email addresses.  It's a standard best practice.  If we did, we would have a high risk of being labeled by ISPs as a spammer.  But again, the number can be significant, and has caused confusion in the past.