To put it another way, if a student receives an 85% on a test, and the average score of all test scores is 85%, then the Z score would be 0 because the student’s score is identical to the average.

It shows the actual distribution of Z-scores (in red) and the expected normal distribution of Z-scores (in gray). As we see, the Z-score distribution is skewed heavily to the right.

Population Study Published: 02 September 2015 LMS tables for waist-circumference and waist-height ratio Z-scores in children aged 5–19 y in NHANES III: association with cardio-metabolic risks ...