I turned Oxford down for Imperial, the course at Imperial was different and I preferred London as a place. One of my best friends turned down Cambridge for LSE for law last year. Also know a boy who turned down Cambridge for UBC in Canada. It's not uncommon, but given that Oxford and Cambridge are the best overall universities, the vast majorty of those with offers will take them. But as somebody said above many people with grades and intelligence suitable for Oxbridge don't apply, and go elsewhere.