how many advantages they really taking when come in the selection process against those not doing science subjects ?
I think people with a scientific academic background have a distinct advantage over those without. KA are very emphatic on numerical ability, as has been demonstrated by the quality and profiles of people who have continually progressed through the selection process. If your academic/work background makes you deal with numbers on a regular basis, you are sure to perform much better under pressure, as opposed to people who don't use mathematics as frequently. Just my opinion.
But in case you wondered, they still allow people who don't make the academic prerequisites to take the Initial Test. I was surprised when they invited me for the test, even though my academic profile doesn't quite match what they seem to want.