Dear Aspirant, 

Top 20 percentile is not something which government or board could decide. Percentile is the calculation of how many students have performed well over how many students. Consider the following example.

There are 100 students and out of the 100 students, 20 students scored 95% and 50 scored 60% and the rest are failing. Then the criteria for top 20 percentile is the student having equal to or more than 95% despite being the 30% of the students not able to get the passing marks.

So it depends upon the performance of the students which decides the top 20 percentile.

Your percentile shows from how many students you have performed well. TOP 20 percentile indicates that the score of these students is greater than 80% of the students appeared for the exam.

Hope this helps.

Thank You.