Ultimately it probably boils down to competitive qualifications in the group of people interviewing that day (not withstanding the other details that go into choosing a candidate).

Let's say 12 people interview that day. All the interviewers might get together at the end of the day in a round table fashion and discuss the candidates. They might decide that 10 out of every 12 will make it through, 2 will receive a no thank you. They repeat this process every day.