Quiz Repeating

My student keeps encountering the same quiz more than once. Why is this happening?

If a user does not pass a quiz (scores under 70%), the quiz is placed back into the pool of quizzes that could be randomly assigned to the student in the future. Only passed quizzes are removed from this pool. By repeating a failed quiz, the user has a second chance to gain points on that quiz, and it helps the user to build up the content knowledge that he or she may have lacked the first time around. The quiz will not reappear immediately. Because passage assignment is random within each reading level, it may be several days or weeks before the quiz returns, but it always remains a possibility.