Banding is very complicated but here goes at explaining how it's used in Lewisham - with apologies if this is out of date in any particulars but this is how it used to work.
Lewisham community schools (a dwindling band as academies take over) and some of the voluntary aided and voluntary controlled schools used to use banding like this: all children attending Lewisham primary schools took a banding test at the end of year 5. When all the tests had been marked the children were ranked in order of their total scores. Very roughly, the top 20% were classified as Band 1A, the next 20% as Band 1B and so on through Bands 2A, 2B and 3. Each school said in its admissions policy that if oversubscribed (and they always are nowadays because of the way the secondary transfer system works) it would take 20% of its intake from Band 1A, 20% from Band 1B etc etc. This should mean that the school ends up with an intake that more or less matches the ability profile of the area as a whole. (In practice that doesn't always work out but that's the theory.) It also means that if they are more oversubscribed for Band 2A than for Band 1A (say) you need to live nearer as a Band 2A applicant than a Band 1A applicant to get a place. Many families find this very confusing.
Aske's uses banding in a different way. All applicants take a banding test in autumn of year 6 (a different one from Lewisham's). When the scores are in, applicants with a score of between (say) 91 and 100 are classified as Band 1, 84-90 as Band 2, and so on down the line to Band 9. (I've no idea what marks represent the actual boundary points between bands, just that there are 9 in total.) They then tot up how many applicants there are from each band.
Say there are 200 places available and 2000 applicants in total, of whom 500 have scored in the Band 1 range. That's one quarter of the total number of applicants. Aske's would then say that of the 200 places available 50, ie one quarter, will go to Band 1 applicants. If 50 applicants were in Band 9, they'd save 50/2000 places for Band 9 applicants, ie 5.
As you can probably see, that means that if Aske's is swamped with applications from higher ability children (which it usually is) its intake will reflect that because they will get more places. Of course, if it suddenly got a lot more applications from lower ability children the reverse would be true, but that hasn't happened because, I assume, parents of lower ability children tend to assume that Aske's isn't for the likes of their children, especially if they live more than a few metres from the front gate. Whereas the higher ability children's parents will apply from miles and miles away on the offchance...
This is a contrast from the Lewisham position because using its banding policy they will never take more than 20% from Band 1A.
You'd have to check, but I believe Prendergast Hilly Fields uses banding in the same was as Aske's does. Not sure what all the academies do.
Hope this helps and good luck!