I mentioned the difference in attitude at Cambridge a few years ago in this thread: Oxbridge admissions for Americans
In summary, Oxford is more prepared to make offers to Americans that are declined, whereas Cambridge hates that. You can look at the difference in criteria (and the way testing is done and offers are made - for example Oxford shortlists from tests before you go for interview, Cambridge makes offers that they know 20% of applicants will miss) at least partly as Cambridge being less accommodating to Americans.
In addition, Cambridge might well make any offer conditional on your Physics AP score (I doubt they would ignore this gap, especially without 5 APs in hand), whereas Oxford might be more accommodating and make an unconditional offer if you have a good PAT score and interview.