Although it is counter-intuitive, but a perfect score can lower the student's average when weighted grading is used.
For example, a student has an average of 105% in the category for quizzes. On the next quiz, the student scores a perfect 100%. However, this score is lower than the student's current average for that category, so the category average drops, which then lowers the student's overall grade.
You can experiment with how this works with the weighting calculator.