Say there are 10 people in a class. if out of a test of 100 marks, the person who scores the highest marks scores 99, his percentage score is 99%. the person who scores 90 and is scores the second highest has a percentage score of 90%.
In a percentile scoring system the the person who has scored 99% being the highest in the class becomes a benchmark for the comparison of other students in the class and therefore gets a percentile score of 100%..Then percentile is calculated for every student assuming that a score of 99 out of 100 is 100 percentile.
So the student who scored 90 now will have a percentile of 90.90. Because the highest score in the class was 99(With a percentile score of 100)