In IIT Council meet, IIT council agreed on top 20 percentile as eligibility criteria for admission to IIT, final decision is pending but it is almost approved, major changes are now not possible. IIT council will give final words on this agreement only after IIT senates meeting.

According to data from the National Institute of Science, Technology and Development Studies, in 2010, 29,000 students appeared for the ISC (science stream), while 600,000 sat for the Uttar Pradesh board exams (science). So only 5,800 (20 % of 29,000) ICSE students will be eligible for admission to IITs, while from UP, 120,000 (20% of 600,000) will be eligible!
In 2009, Indian Institute of Science (IISc) Bangalore scientists PS Anil Kumar and Dibakar Chatterjee compared the performance of students from India’s 29 boards over 10 years in a common examination. Their results clearly showed that CBSE, ICSE, West Bengal and Andhra Pradesh board student does much better in science than the other state boards student. So it is clear that ICSE student does much better in science than the UP board student but their chances of getting admission in IITs are much less as compared to UP board.
So with this top 20 percentile eligibility criteria, the better the board, the less chance you have of getting into IIT. The bigger the board, the higher your chances.