Basically, Oxford's admissions process relies far more heavily on pooling than Cambridge's. Whilst the Oxford admission process is rather slick, Cambridge's is still relatively mediaeval; people have to wait weeks to find out their results, and sometimes even have to return a month or so after their first interview for another one, if they're 'pooled' elsewhere.
Obviously being pooled is better than a straight rejection, but it must be a fairly horrific experience, especially if you're turned down having been filled with a second pang of hope. The ideal solution is that, if you apply to Cambridge, you think long and hard about the college you pick. Whilst at Cambridge one's choice of college is fairly open (at Oxford most colleges have prohibited subjects), the pooling system makes college selection crucial, particularly with the sciences, where college reputation fluctuates.