As to people who are admitted then not doing brilliantly. Well there are all sorts of possibilities.
I am probably talking about myself here. I was physically and emotionally abused in the house where I grew up, and assumed that when I went to university this would constitute escape.
Which on one level it did, but I had to go home in the holidays. And being among lively intelligent people at university made the oppressive silent atmosphere at home still worse.
This was in an era where pastoral care was less good than it is now, and the abuse was simply not young people's radar. There were relatively few mixed colleges, and even in the ones there was, there was a very masculine atmosphere.
Having grown up in a very peculiar atmosphere, I could only deal with aspects of collegiate life - which I think was very much an extension of boarding school life - by wearing a mask. Which cracked sometimes.
I did get a lot out of my studies and didn't do badly. I just know if there wasn't so much I was struggling to contain, I might well have done better.
So I'm not sure about the comment some people with personal/mental health issues manage not to let it impact their academic performance
I think it's not a matter of a lack of will-power and determination. I had those - and frankly without them I'd probably have ended up dead in a ditch long ago. But I also feel that with a more stable and nurturing background, I would have been able to approach my studies without having to constantly shut the door on massive unhappiness.