Its often the intensity of Oxbridge that is the problem - 8 week terms and 'full-on'. Some people perform better with 10 week terms and not the same psychological pressure. This has nothing to do with intelligence - its about learning style.
Most people who make it to Oxbridge have often been high achievers all their lives, and getting to a Uni where everyone is as clever (or cleverer) than them, where they may get mere A grades instead of A*s, that too often can be an enormous dent to their confidence. Its not uncommon for students to find this very difficult to get their heads around.