I discussed "Why is it worse to get rejected from Oxbridge than anywhere else?" with DH this morning (we are both Oxbridge). We decided that it's definitely partly bullshit prestige - fifty years ago there was a huge gap between Oxbridge and ... everyone else, but that really doesn't exist any more.
But maybe part of it is because of the interview system. Getting rejected from somewhere that's never met you can be brushed off; getting rejected from somewhere that interviewed you for hours is more personal.
As an aside, although it's definitely valid to point out that a practically flawless record can end up being ignored if that one last mark evades you, it's worth remarking that her final rejection will have had far more to do with the other candidates than her own self.
A college just doesn't know how many of its candidates will actually make their grades. Say they have ten places; for logistical reasons they want ten people to fill them, but statistically one or two will miss a grade. So they make eleven offers.
Come results day, six make their grades. Three slip by one grade, two crash and burn. The three ring up in a panic; the tutors review their paperwork and say "you know what, this candidate is genuinely still good enough, and we've got enough wriggle room - she's in."
OR
Come results day, ten make their grades, and one misses by 1 raw mark. They have ten places and ten offers to honour. No wriggle room.
I've known plenty of people miss their grades and still get in - because they fell into the first situation and not the second. That has far less to do with their results and far more to do with everyone else's. They might have absolutely loved her, but if they don't have a place they don't have it.