It is not a breach. And for external applicants it can be a bunfight in August itself and a negotiation as to what A levels you are offered. Can also happen to internal candidates if they do not meet the threshold for some subjects, but meet overall point threshold. Nobody is going to let you study Further Maths A level at a competitive grammar without a 9, if that course is massively oversubscribed, but typically internal candidates will get preference. Most of the internal candidates also already did at least Further Maths GCSE or sometimes even an AdMaths qualification.
Let’s take a really competitive grammar ranking based on top 9 GCSEs - so if you get 9x9, 81 points, then on results day you will get your top 3 A level choices or 4, because some grammars start with 4, or IB Highers or whatever. If you get good grades but not the top, you may get an offer a day in, over someone with slightly better grades, but different A level courses.
They will prioritise internal candidates for eg Further Maths and Computer Science first, so if you want to do English, German and French, History you will likely get in with much lower grades than someone going for the most competitive courses in a top boys grammar school. Then there are conditional offers based on your predicted grades which have to be evidenced by a school report for some schools, others no evidence of a school report, but mocks etc - some offers based on results in August only. It is a on a case by case basis, the schools themselves handle Admissions and none of it is a breach.
This August is going to be interesting, to say the least, with the private school VAT. Some of our local schools have had 1500 applications for 60 odd places.