Moodle version: 1.9.6 (Build: 20091021)
Grades selected for column averages set to 'Non-empty grades'
Graded roles as enrolled student and manually assigned student.

If a student has multiple roles, the averages are counted multiple times.

John & Jeff got 50% for the quiz and James didn't get anything.
John is an enrolled student and manually assigned student.
Jeff & James is are enrolled students.

Average was 50 * 2 + 50 + 0 = 150 / 1 = 150.
Average should be 50 + 50 + 0 = 100 / 2 = 50.

Reason for difference is the scores were added multiple times to the total and non-emply grades were less because the count of students with graders was greater because of the multiple roles. ie: 14 students - 9 grades = 5 non empty grades when it should be 6.

Fix attached.