Because some schools can get a full refund within three weeks for no reason (because courses are often three weeks, such as Macquarie University). Some schools give more time, five weeks (such as Newcastle University). But it is also possible that UNSW has only a two-week time limit. Secondly, if you drop out of school for special reasons, even if the deadline is exceeded, the school may refund the money in full (such as accident, serious illness, unreasonable cancellation of the visa). Finally, some schools refund money according to time. For example, full refund within 3 weeks, half refund within 7 weeks, and no refund after that (Macquarie is like this).