<p>On the differing acceptance rates by subject, a lot of this is due to shifts in the popularity of subjects at A-level amongst UK students which aren't reflected in changes in the size of Oxford's academic departments.</p>
<p>An obvious example is Classics, which is still a major department at the university, but the teaching of Latin and particularly Greek in schools has declined massively and is now rare in the state sector. To a lesser extent the same probably also applies to many of the sciences; articles bemoaning a decline in the number of students taking sciences at A-level are not uncommon in the UK press. </p>
<p>It is quite possible to enter for many of the humanities like Law or PPE or even History without studying related subjects at A-level, but I think it would be much more difficult, if not impossible, to take on university level Chemistry or Physics without the relevant A-level/IB etc.
All of this narrows the field of potential applicants. </p>
<p>For a lot of the combined subjects the numbers are really too small to read anything particular into them.</p>