In the list of quizzes in the Quizzes tool, the total number of attempts and submittals made by all students is listed by each student under the "Grade" column. However, after a student who made any quiz attempts is unenrolled from the course, the counts are not updated (decremented) to reflect the new course roster.
If you go to grade the quiz by selecting Grade by the drop-down, you see that the attempts from the unenrolled students are correctly deleted. However, now the number of attempts in the "Grade Quiz" list does not match the count in the "Quiz List".
This is a known issue.