IIM Ahmedabad puts a lot of weight on past academics. You need to have a consistently good academic record in order to get a shortlist. Students with diverse backgrounds and 90+ percentile from 10th onwards have a good chance of getting a shortlist, even if their CAT score is comparably lower. Students from non-engineering backgrounds have a good chance of getting into IIM Ahmedabad with a lower CAT percentile.
The brand value and the reputation of the college are very high and the college tries to maintain that by enrolling good quality students. This is done by setting a high cutoff in the CAT examinations.
In order to get 99 percentile in the CAT examination, you will have to score 70-75 marks in each section and the reserved category will have to score 50-65 marks in each section. You also need to have a good academic score in order to get admission at IIM.