I think, up to a point, it is systematic, but once they've reached a pool of applicants, it becomes more "are they good enough?" which depends on the luck of who is determining if the person fits the bill.
For instance, UC Berkeley does follow the example to considering the top 10% of students in the state of California based on their scores(because thats what the government says they have to), and an extensive point system which makes the applicants comparable by numbers but also more than just tests, etc . However, once they've selected that range of applicants out of the entire bunch, it becomes much more about "can this person suceed at our uni? Are they what we need?"
So yes, Oxbridge/HYPS seem like a lottery and sometimes very random in their selections, but I think that they know what they want, and while they sometimes may be wrong, they rely on both the formula and the deeper side of what the person's standing actually means.
Not sure if that is reassuring, but yeah. lol