The exam boards try to make marking as mechanical as possible. But there is always some subjectivity, even for maths. There are marks for SPAG (spelling, punctuation and grammar) and other bits and pieces across the spectrum. Does an examiner miss one typo and you get 5/5, or do they mistake a letter and give you 4? Where schools systematically under- or over- mark makes a difference, and that is controlled for.
Someone once told me that you need an element of luck in key milestones in life. A shared hometown with your interviewer, a grand prix driver who gets a better nut, or bumping into someone in a nightclub who you find as the best spouse ever etc. But what I can say is that as somebody who has marked these papers, all luck on exam papers is diluted down to as low as it could ever be. In anything. Sample checks on samples, checking marking of coursework against actual results etc. Taking all detail about a students name, sex, location out of papers etc.
The reality is that the results seen do have some volatility year-on-year and student on student. Some schools are better than others. Some years are better than others within a given school. It is perhaps testament to the quality control of the examiner that the luck is more about the cohort within a given exam than the markers of the papers.
Take the longer view. If you are an A student in a given subject, you will on average get As across GCSEs, AS levels, A levels and a 2:1 degree in that subject. If you are marginally into the A band, maybe you get a mix of A and B grades, with a bias to A. Perhaps your daughter was lucky to get into Cheltenham but unlucky to get into Cambridge. But these kinds of risks are as low as they can ever be.
One piece of advice for any student:- don't be a 'smart Alec' and write at the end of the paper "wishing you a good summer (hope this doesn't mess it up too much)" or "thanks for marking this paper". These comments will attract negative prejudice from just about any third party who looks at them, and makes a borderline mark go one way or another.
Best of luck to all A level and GCSE students in their lives !