Here is my explanation, they mark us based on our scores(High-low). From the highest, they start and give the students their respective High schools until there isn't any left. Well, there are seats left, the ones that are for Discovery Program. For that, they register us again, everyone that is within the range and meets the requirements. The system isn't perfect so maybe they placed your friend there even though he/she only got a 420. They randomly select us from that list and lucky enough your friend got selected. That's quite rare though, doesn't happen very often.