Hi Aspirant,
Yes it is a good question because most of the time we see that the the students of central board gets a higher percentage than the others. The reason is because many times we see that the papers which are generally made by central boards are little bit easy as compared to state boards. In case of state boards it becomes tough. Even the checking done in cbse and icse is not so much tough, but as we compared the checking of state boards it is tough that is why the passing percentage of State board students is low as compared to the cbse and icse.
I hope this answer will clarify the reason of your answer.
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