I was talking to some physics professor from bristol the other day (i dont know what his name was but he was at a meeting at the place i work), and was interested that he said lots of departments will automatically reject almost all applicants before 15th october because they dont want to be used for "oxbridge interview practice." (although apparently physics don't do that because they don't get enough good applicants.)This may explain why so many good candidates are rejected from there, if you are really serious about wanting to go there it must be best to apply after the oxbridge deadline.