You should check in your quiz wether a penalty is set at each question.
Penalty means, that the student gets less points the more attempts he needs to answer the question correctly.
The quiz settings determine wether a penalty is active, the question settings determine how many points a student can receive and how high the penalty in points is.
Suppose you have a quiz with one question that has 1 point an 0.1 penalty.
Your student goofs the first attempt: 0 points, 0%
Your student succeeds the second attempt: 1 point minus 0.1 penalty = 0.9 points, 90% (instead of 100%)
A second student fails two attempts and passes the third one: 1 point minus 2x0.1 penalty = 0.8 points, 80%