These are the students who enroll in a class (or classes) during their first session after registering with edX, and who have not (yet) responded to their activation email to confirm that the email address works.  And may never.  They just registered to look around at the course, and never come back.

My concern was just echoing that of others -- that there is confusion when these numbers don't match.  I believe that for CS50, the number was pretty darned high, something like 10000.