I think it's more complicated than that.
At a basic level, you need higher grades to get into Bristol than you do to get into Bolton, so employers make certain assumptions about the kind of students who go to Bolton. This is probably even more true now students can use clearing to get into a better university if they exceed their expected grades, as opposed to a few years ago when clearing was only for students who had missed their grades and needed to find a university that would take them with lower grades.
In terms of the quality of teaching, academics also tend to want to gravitate towards more prestigious universities, so it's possible that the quality of the teaching is better. This one I think is less clear cut, because many academics work their way "up" until they get the position they want at the university they want, where they then stay until they retire. I went to Bristol to study a competitive subject, and not all the lecturers were as high quality as I was expecting. I also remember an excellent young lecturer who resigned very suddenly at the end of my first year, after many of us had signed up for the module she was teaching the following year and bought the books on the reading list, because Bath Spa University offered her a full time post on a better salary. Those of us taking her module ended up being taught by someone absolutely awful, who had been dredged up from God knows where at the last minute to fill the vacancy.
I definitely felt that Bristol was relying on its reputation a lot, and that that reputation wasn't necessarily deserved.
So I think it mostly comes down to the perceived quality of the students, and as a general rule, it is true that students with higher grades and better CVs will be offered places at the "top" universities, whereas universities such as Bolton are much easier to get into.
What this doesn't account for is the fact that some students will need to go to university in their home town, perhaps for financial reasons or because they have caring responsibilities, and some students are simply poorly advised. A student from a working class background who is the first person in their family to even think about going to university is probably not going to be as attuned to university rankings. They won't necessarily know whether Bristol is better than Birmingham or Brighton. And there won't be anyone in their family telling them not to apply to Bolton because it doesn't have a good reputation. So they will be stabbing in the dark a lot more. And when they get an offer from Bolton, their families and friends are more likely to say, "That's great, well done!" rather than, "Really? Are you sure?"
I would still expect the brightest and most driven students to have enough wherewithal to research universities properly themselves. I think you're highly unlikely to find a really exceptional student studying an academic subject at Bolton. But I do think there are probably a fair number of decent students at universities like Bolton, who are just as good if not better than some of the more average students at Bristol. It's just that those average students at Bristol had parents who send them to private school or paid for private tutoring and hockey club and flute lessons and hired someone to write their personal statement and made sure they applied to Bristol and Durham and St Andrews rather than Bolton and Derby and Wolverhampton. That's where the unfairness comes in, I think.