Think it boils down to the whole element of univeristy prestige and how it's perceived to be that if you're not studying at a Russel Group university, you're doomed to utter failure and may as well give up now - not just from the opinionated "league tables are the god" twits on TSR, but to some extent schools as well - they can be very pro-reputation/league rankings (I know mine was).
So the continous debates on here regarding reputation, ranking and old but ever used RAE report fuels the question every week or so when it's asked. I know I asked it a couple of months ago, purely out of interest, but also to back up my own feelings about the subject (coming from a small relatively unknown uni) that when it does come to PG courses, it's just not all about where you studied for your UG.
Regarding Oxford and Cambs, it's not until you hear the experiences that say the_alba has experienced for her Mst, or when you get told by your own head of research that some of the worst PG students he had ever had, had come from Oxbridge, that you do begin to realise that the stereotypical reputation of a university doesn't always live up to the every day scenario.