They're likely to be able to guess you're Oxbridge if your form shows up on the 14th of October (I know mine did!) and you have straight A's or similar.
This
might make them less inclined to make you an offer, because the
assumption is that if Oxbridge accept you, then you won't be going to Durham. This
may mess up their intake figures - they
could end up with too few people
if lots of offers are made to Oxbridge applicants who then get into Oxbridge and go.
If they account for this by making lots of offers to Oxbridge applicants and overfilling their quota on the
assumption that many of the people won't come anyway (but they want to get as many as they can), then it
may turn out to be the case that hardly any of the Oxbridge applicants get in and they all go to Durham (
likely to be their second choice) - Durham will then have more people than they can house / fit into lecture theatres but they will be legally obliged to take them.
Perhaps in the case you mention they're holding back offers until they see how many good applicants they get - "maybe lots of stunners will come in after the Oxbridge deadline and we won't need those rah tw*ts". Jokes.
Please note obvious use of words in
bold - this is just how they
might play it...do we have any evidence that they are slightly more hostile towards Oxbridge applicants? I know Bristol's hinted at it, but I was having a discussion with a teacher at my school t'other day about how Durham try their best to make sure they get a lot of the Oxbridge applicants - after all, even the Ox-
rejects are bloody good grades-wise and it's kind of a lottery regarding who gets in there anywayz. So by bagging Ox applicants Durham can get some of the top students in the country. That was teach's take on it anway...he's very knowledgeable so I chose to believe.
ZarathustraX (Soon to be Oxbridge-reject en route to Durham

)