I have secured 96.25% in boards(ISC) but still haven't received an offer from NUS whereas a frnd with 96% has.?

Grades are important, but not enough to gain admission. Other considerations often include things like specific courses taken, class standing, which institution you attended (they have varying reputations), letters of reference from teachers and others, character, leadership (e.g. elected class offices, etc), community service & volunteer activities, sports participation, awards & competitions won, talents (e.g. music, arts), and more. That's a minimal difference in scores, but you quite likely have big differences elsewhere. Each person is different, with different skills, abilities, talents, experiences, character, personality, etc.