Just look at it from an employers point of view. 100s of applications to read through, little time, 100s people same age, school, then uni some employment some extra curricular how do you choose who to interview.
Bradford students - average or less than average Alevels why else would you be doing your degree at Bradford unless you were a student living at home. 2.1 from Bradford quality of that degree is pretty much an unknown quantity
Oxbridge - 4 As + probably, 2.1 Oxbridge had to interview for a place and Ox applications are amazingly competitive so you know that they interview well. Ox degrees are known not to be for slackers, hasn't got the party reputation of some other unis. You know that this person is among one of the elite intelligence wise in the country. Even if the other has more specific skills you know that someone from Oxbridge has a quick brain and can pick things up quickly or else they wouldn't have been in Oxbridge.
You can only interview one, which would you pick.
It stands to reason that Oxbridge have higher employment rates.