Every applicant can apply upto five unis so not all the 500 offer holders would firm or insure UCL. For those who firm UCL there would be some could not achieve the required grades. For the insured, some would goto their firm choice unis and some might miss their grades. So eventually there would only be a fraction of offer holders who could get into UCL. Only UCL knows the proportion and give offers accordingly. Of course UCL would under-provide a bit to avoid over-enrolment. It happened during COVID when A-Level exams were suspended and results were given by schools. There were "surges" in high grade students and some unis over-enrolled, Durham being the classic example.
In case the number of firmed and insured could not add up to the total vacancy, UCL would take up some near-missed.
I guess most unis would do the same.
All my wild guess.