The student number controls were set up originally by government to limit the costs of university places. Rather than an absolute requirement, they allowed the government to fine universities if they took on too many students.
When government funding was largely taken away and students asked to pay £9,000, this was supposed to be part of a bigger package of measures in making universities function as a market. But it was felt that removing the number controls completely was too financially risky, so they only did it for the top grades (AAB first, now ABB). There was always a bit of flexibility as to what qualified (eg AAC, BTEC etc).
This means that if you get those grades, there are no potential fines, so universities can recruit as many students as they want too and/or have room for. If you get below ABB, there is a set number of places that can be offered without a fine (this bit is at an institutional level, so over or under recruitment can be shifted across depts).
To be honest, the way we handle admissions is pretty uncertain from a university perspective. I have to estimate how many people that I offer to will make us their firm, what % of predicted grades will be correct or above, and how many other institutions will accept/reject places where we are the insurance.
On past estimates about 25% of people I offer places to will make us firm and get their grades. So I have to make 400 offers to fill 100 places. These offers are all legally binding, and if they all came, we would be in serious trouble.
So the extent to which universities can be flexible depends on the successes and failures collectively of the people that have applied.
There is also a lot more flexibility in classroom based courses than lab-based ones. It is much easier to put some extra chairs in a lecture theatre or increase seminars by 1 or 2 students than create extra bench spaces (much cheaper as well!).
So generally speaking if you get AAC rather than ABB on a classroom based course, you are likely to be accepted. If lab-based, probably, assuming they don't have too many other applicants getting top grades.
If you have an ABB (or above) offer and you drop to BBB, you might be in a worse position than if you were made a BBB offer. Universities that don't have many controlled places have to take those they offered BBB to, but can reject those who missed the grade.