I've never gone through the system, but my first guess would be it's probably so that all the slots aren't given to kids from ideal, wealthy homes, raised with tutors and computer camp, but that there's some room in there for kids who might be really bright, but also dealing with effed up home situations that knock a couple points off their standardized testing. Or who are bored as fuck in their current school but would really come alive at a CS-focused school where they can learn alongside the most qualified applicants. Because school isn't a consulting firm or a hedge fund.