All schools have to accept that they are not first choice for all applicants. There are lots of very good academic girls schools. There might be one or two which are seen as the absolute top tier, but they won't be everyone's top choice, often because if geography. Once you get to the extremely good but next tier down, those schools know that people applying to them are probably applying to 2 or 3 or 4 more. Postcodes might give guidance on top choices, as might siblings or asking people, but people make all kinds of choices which are hard to predict and will drive past another great school to get to their chosen one, which just seemed to 'fit' their child. All of the London schools will be struggling with judging making offers and will have years of a bulge class or having to go far down the WL or beyond it, because things don't turn out as they predicted. A school like NLCS has space for a bulge if it comes to it. Of course, they don't say much about these things, especially using WL because a big part of their appeal is people's beliefs that the school is taking the very best students and that its most people's top choice. Many parents are pretty sure the school they want is most applicants top choice - it might be for the handful of other parents from their Prep school that they have spoken to, but it's very difficult for parents or schools to know and when asked, parents are not always honest or open about these matters. Schools try to gather the info from application forms, from interviews and asking the children often inappropriate questions, but they still often don't get accurate answers.
Interstingly, I knew one popular girls school last year, who in September had to spell staff that there were no academic scholars in the 1st Form - they had made plenty if academic scholarship offers, but all of those girls had chosen to go elsewhere. It's a popular school which does fill or close to fill its year group, but is has to offer 3 or 4 times the number of places it has in order to do that - it simply isn't the top choice for probably 3/4 of applicants, but a back-up. They fill from those who do have it as first choice and from those who didn't get their first choice. And you'll find too, that many of these schools get close to filling but not completely and then take a good last minute applicant who has just come back from abroad or wherever, maybe in May or June or even later. Even some of the most popular schools will do this.
Perhaps it's hard to feel sorry for these schools with their struggles and uncertainties when they are charging vast fees and don't seem to be hard-up. Fair enough. However, exploding offers are a sign of the difficulties within the system. The level of the difficulty does vary from school to school. Being really popular reduces the risk on one level, but also makes even more people use you, many as a back-up. Being in a location many can reach, even if not easily, again makes more people choose you, but for many as a back-up. This is the very anxious time for schools now - because the numbers they attract and the quality of those candidates will have a big impact on the financial strength if the school and it's academic success at GCSE and A level in a few years time. Every year group isn't equally able and yes of course they should be able to do good things with any ability, but the bottom line determining academic success is how selective it is on entry ...and a lot of it depends on whether the WL has had to be used and by how much. You can see why schools are keen to avoid it if possible.
I think that the system which worked for many years when people applied to a couple of schools, might not be workable for much in days where many apply for 5 schools, creating huge uncertainties.. Schools breaking away and doing their own thing and looking for ways to reduce the risks of not knowing how many of the vast numbers applying are actually serious about attending, is a sign of this. Maybe a new, better co-ordinated approach will emerge or perhaps more and more schools will do their own thing.
It's a peculiarly London thing because of the urban nature and existence of many schools close together, plus the ratcheting panic which seems to grow amongst parents and drives them to apply to larger numbers if schools. Even a few miles south or north in the outer zones or the Home Counties, schools can use the traditional system with much more certainty because schools are spread that little but further apart and the numbers applied to not quite so high.