We created a Quiz using the 'Advanced Scoring' option and insert negative bounds for some questions (e.g. -8 / 8). After completing the quiz it seems we get the right scores but the percentage does not seem correct (as if the system always gives a little bit more).
The calculation of quiz using negative scoring might seem confusing. Below is the example of how grade is calculated for such quiz attempt which should give you the idea.
Imagine we have a quiz with the following questions:
8 questions with points from 0 to 15 = each weighs 15
3 questions with points from 0 to 4 = each weighs 4
4 questions with points from -1 to 5 = each weighs 6
1 questions with points from -8 to 8 = weighs 16
Quiz total points are 172 i.e. a question from -1 to 5 points equals to 6 points, and a question from -8 to 8 equals to 16.
In my attempt to complete this quiz I answered correctly to all questions that weigh 15 and 4 points, and incorrectly to all questions that have negative scoring:
8 questions * 15 points = 120 points
3 questions * 4 points = 12 points
Total 132 points
4 questions * -1 point = -4 points
1 questions * -8 points= -8 points
Total -12 points
That is, my score shown in the attempt is 132-12 =120 points
Next, the percentage is calculated from the positive answers you scored. That means my grade percentage is calculated from 132 out of 172 points, and that is 76,7%.