All independent schools "over-offer", that is, they might make 200 offers when they have only 100 spaces to fill. They do that because they know that they will not be the first choice of every applicant that they offer so, making 200 offers, they expect only 100 acceptances.
When they are overly optimistic about the "yield", that is, it looks like fewer than 100 offers will be accepted, they go to their waiting lists to get to 100 acceptances.
When they are overly pessimistic about the yield, that is, it looks like more than 100 offers will be accepted, they can do several things:
- Increase the size of each form, so that 5 forms of 20 may become 5 forms of 22 or 24, provided that facilities permit.
- Increase the number of forms to make a "bulge year", so have 5 forms of 20 become 6 forms of 20, again, provided that facilities and staffing permit.
- Withdraw outstanding offers once the acceptances reach a hard limit of number of places when facilities and/or staffing no longer permit.
You'll find that most schools, in the small print in the offer letters, reserve the right to do option 3 in case they really misjudge the number of offers and they absolutely cannot increase the number of places further. When a school withdraws an offer before the decision deadline, that offer is said, colloquially, "to explode".