I think it is quite simply, as you say, that those with an A star cannot get a higher prediction. Yes, lots who are predicted them do t get them, but all the very top students do get them and we’re predicted them.
Re why student wait so long for offers (or rejections) including those with offers in hand….I think it’s all about unis gradually getting more accurate info about who will actually be taking up a place, along with the fact they are running different lists for home students, contextual and foreign students.
They have an idea based on previous years of how many offers they need to make in each of these categories, to fill all their places, without ending up with too many students that they cannot accommodate (keen to avoid too many after difficulties of Covid and teacher grades resulting in this). There is the fair consideration deadline of the end of Jan, before which unis are meant to not make too many offers so that those applying a bit more later are not disadvantaged. Because this date exists, until it passes, unis cannot know if they have yet seen the best applicants they will get. So they have to wait until then. Perhaps they will make some offers to students in each of the home, contextual and foreign who have absolutely top grades (all A star) but some places also want to look at PS too. Until the end of Jan passes they aren’t in receipt of all the applicants (and even then some others might appear). After this, they can start making offers to each category of student and usually to the best first and then downwards. It they won’t make all their offers yet.
Then we get to the point where students start firming and insuring. Some unis try to push offer holders to choose the, by opening accommodation bookings early and before all unis have made offers. They aren’t mean to but some places do to attract people to take the bird in the hand and get accommodation, rather than risking a rejection elsewhere and having missed the best accommodation. At this point, as unis find they have been insured or rejected by students who hold offers, they are able to make further offers to their more marginal candidates in each category. Some years more will firm them and then they can’t make further offers and some of those more marginal students have to be rejected to avoid over offering. Other years they find more haven’t accepted or have only put them as insurance. They know roughly what percentage of insurance candidates will actually come, and on that basis can make more offers. It can almost be a certain number of rejections from students choosing other places to firm or insure, leads to a certain number of offers. Some places also find by delaying offers, some candidates might drop out and firm somewhere else to get accommodation. What the uni really wants is to know who are the best candidates that actually prefer them as a choice and will actually come. Until candidates firm and I sure, they do t know for sure how many might have an Oxbridge offer and prefer that…..that might mean a lot of good offer holders are no longer such likely arrivals.
The thing I think that complicates it, is the running of different lists and having to co-ordinate them. Foreign students bring more money…but often those who apply don’t actually come. Students with contextual offers are really wanted, but there’s the question of how many with lower offers should be taken and also the risk that sometimes those with lower predictions who only get one because they meet the lower contextual offer on application, might not achieve it. At that point, will the uni take them anyway with what could pretty low grades? Because if the different lists, someone predicted 3 A star from a private school might find they don’t get offers as that list is moving at a different rate and someone with one A star and 2 As might get offers from a state school contextual or foreign applicant. Unis probably know the likely firm/insure rejection rates from each of home, contextual and foreign students. They also know from each group what the likely achieving of the offer rates are too from firm and insurance candidates. All this needs to be factored in and only as it is, can decisions to make more offers, make rejections or hold candidates for more information be made. In the end it’s demand and supply. The only thing anyone can do is make themselves the best applicant they can be….the higher the predicted grades and GCSEs and better the PS, the more desirable a candidate should be within the home state, home private, contextual or foreign list they might find themselves. Those with grades in hand might well be kept waiting unless they are 3 A star, because others who are better might appear. And even within these lists and possible categorisations according to strength,there is probably an element of randomising of candidates. Perhaps they look at those with 2 A stars and take 10 to make offers to…but which 10? There will be an element if luck too at the levels where courses are vastly over subscribed with op notch candidates. Fortunately, most courses can’t be this choosy and for most courses, if you meet the standard offer in your PG, you’ll get an offer.