To recreate this issue:

1. Add a new assignment (mod/assign, not the 2.2 assignment) to a course with enrolled students.
2. Log in as a student and submit the assignment.
3. Log in as another student and submit the assigmment.
4. The Grading summary on the assignment page should correctly reflect the number of enrolled students and the number of submissions (2).
5. Unenrol one of the students who submitted the assignment.
6. The Grading summary now correctly shows the number of enrolled students reduced by one, but the number of submissions is unaffected.

While this problem is not fatal, it does cause confusion for teachers who look for submissions that aren't there. The problem is made worse when a course is reused (old students removed, new ones enrolled), because the Grading summary of an assignment will continue to list the old submissions and there is as yet no reset option for the new assignment module.