Goodness, just caught up with this thread - it's drifted a bit from the OP's question.
I don't know much about Oxbridge admissions processes. One thing though is that they clearly take it seriously enough to devote a lot of academic time to interviewing. Interviews are extremely rare elsewhere.
I'd happily bet a substantial amount of money that there isn't a fixed algorithm. For one thing, colleges have substantial autonomy.
Contextual GCSE and progress scores mean there is more information available to have a sense of student trajectories.
Some variance in student performance will be school-level, some variance will be student-level. (And some will be teacher-level, some class-level....) But there is no bullet-proof way of assessing how much is student-level. Those metrics don't exist.
Imagine that the MFL admissions tutors for College X have 50 applications for 10 offers available, and interview 45 (I haven't looked up application rates, this is just a rough assumption!)
They will have a bunch of guidance from the central university, and also the department reminding them of admissions policy and how to do fair assessment.
They will have college policies too. And undoubtedly mandatory training on how to interview effectively while accommodating needs, e.g. students with disabilities, even if not notified in advance.
The grades are all impressive, the personal statements near-impossible to differentiate meaningfully, some possibly drafted by paid advisers or AI.
Then the interviews, which are intense because you have to be focused, and time is so short. This is followed by collective ranking.
The top handful are really exceptional - everyone agrees. For the remaining five places, imagine that the interview has managed to differentiate 20 really excellent students from 20 excellent students.
Of those they have to work out which five to offer places to, and a few to recommend to the pool.
They do have sight of the GCSE scores - but some of the applicants are Scottish, some are international, one or two are mature students, one or two have first degrees already, and some are applying via the Foundation degree, just a few weeks into the Foundation year.
Imagine that half the ranking meeting is then taken up with one particular case where judgements differed wildly - until everyone realises time is running out and they need to make a final selection.
On balance it's a good thing that there is an attempt at a holistic judgement with human decision-making - which is extremely resource-intensive.
But it should be understood as partly a lottery. That's life though. The labour market is where the fun really starts.
From my experience at other universities, offers are made automatically if predicted grades met the grade requirements. The system as a whole is heavily demand-led now. There was very little human judgement involved - the resources aren't available.