Hi Bill,
Each quiz and exam is set to automatically allow two attempts, regardless of your score on the first one, but it is up to you if you need to use the second attempt. So, in this case, you’ve already scored 100%, so you can just move on to the next unit. But if you take a quiz later and don’t score 100%, you can study some more, ask a question in the forum, etc., then retake the quiz to improve your score.
Does that answer your question?