I'm not sure on what point I've been unclear. No one has suggested that this means of assessment is bullet-proof. No one has suggested that there aren't exceptions.
I was originally just responding to your original point "employers will
always use your
A-Level results to determine that standards at the University". I just don't believe that is always the case, and more than just the odd exception too; there are other means of judging the standards.... such as:
Companies need to use some sort of yardstick, and a fairly good one is that of the grades required to enter into a course.
Exactly - the grades
required, not the grades achieved (which can often be two different things).
Anyway, I don't want to be too much of a pedant so I think we'll just have to agree to disagree

I can appreciate that a high achieving A-level student is more likely to have gone to a top uni, but I don't believe it should be a dominant factor in measuring their standards since a top student would be accepted by pretty much any uni, including those with low standards.
edit: just to give another anecdotal example, yes I know it's just a single exception, but a bit of fun nonetheless!
My wife and I both have honours degrees from the same University (albeit at different times and studying very different subjects).
My A Level results were AAB, so your logic dictates that my uni should be viewed as being pretty good, but not right at the top of the scale.
My wife's were something like EUU(?), which implies that my uni must be pretty terrible.
So two employers looking at our CVs would infer wildly different things from this. They'd be much better off IMO using other means of assessing the quality of the university and courses we attended.