I think that a lot of parents don't get very good advice about Oxbridge. Firstly, they don't realise the academic standard at GCSE or A Level of the majority who apply and certainly those who are successful - because their child might be a top performer in their school which isn't hugely academic or doesn't have a big history of sending to Oxbridge, they think that a clutch of 7s and 8s will be okay, without realising how many candidates are out there with all 8s and 9s.
Then others think it is purely down to the grades - and if their child has all 8s and 9s and predictions of A/A* they assume that must make them a shoo-in. They don't realise that it's possible to get these grades through a reasonable level of intelligence and decent teaching, but these grades in themselves don't show significant aptitude or interest ina subject, and that more is needed for that. They don't realise how important the Oxbridge subject based tests will be for really testing at a deeper level what someone's understanding and aptitude are, or that students often can't accurately predict how they did on such tests. And lots of parents and schools don't really understand how real and genuine interest can be manifested and encouraged - so some students are disadvantaged because they simply haven't been encouraged in this area.
I think of 2 children I knew going for Oxbridge in History. Both had fantastic grades at GCSE and predictions for A Level and indeed gained all A* at A Level. One was really a plodder - not that interested, but listened to advice about what to in exams and delivered. The other had a genuine passion. They read History on the loo, on the train, instead of watching TV and they chose to go and listen to people sharing their views. When the first had this 2nd student described to them, they just could not believe it 'are there people who really do that' they asked, as they asked their teacher to tell them a book they could put on their personal statement, because they knew something would be expected. I think we know which one got the place.
And I realise that it could have been that the great reader with the genuine interest wasn't up to it and might not have done well in interview or in an aptitude test, aren't everything. Perhaps what I'm trying to say, is that people seem to thinking decent set of GCSEs and A Level predictions is enough.....but it isn't, because quite simply there are far too many people with those.