On the thread a few years ago, when my older dd was going through this process, there was a similar discussion. From what I remember, schools basically have the choice of over-offering by lots and taking the risk of having to run a bulge class, or not over-offering much and taking the risk of having to go deep into their waiting list (which means potentially less good exam results in 5-7 years time).
So depending on site size, different schools have different approaches. I remember that PHS had a system (not sure if it still does) where they had a an extra class every other year. So every other year they will offer more. And I remember someone saying that WHS had to put on an unexpected bulge class one year, and that they were being very conservative as a result as they absolutely couldn’t fit in another one, given their constrained site. Though that may have flowed through the school by now. Also that LEH had a big site, and had had several bulge years as they could accommodate them.
They also have different approaches to waiting lists. WHS puts everyone who gets through to stage 2 on the waiting list, while SPGS is rumoured to have a very small one (people were guessing 10). And for Tiffin, there’s a whole (long) thread on the 11+ forum which shows how their waiting list works, with a mathematical approach to working out if a score will be high enough to get a waiting list place before the start of Y7.
In the end, the key thing is that people release offers they don’t want as quickly as possible, so that schools can go to their waiting lists and that then kicks off a cascade of acceptances and releasing other offers. If dd is lucky enough to get more than one offer, we’ll talk about it and maybe book in a quick offer holders tour if there are two we can’t decide between, but release all the others.