Dear Aspirant

In some boards , like odisha board , the marking criteria is quite strict. Thus the top 20 percentile means to be in top 20% of students. For odisha board in 2017, it was 331 out of 500 marks . This means that the person with 66.2% marks was eligible. So you see, if you have scored above 75% marks then you are eligible. But for the sake of students from other board, top 20 percentile was introduced.

Hope this answer helps.

Best of Luck