It seems typical for Oxford to interview you at two different colleges for most courses in most cases, and I think have a somewhat informal pooling system in case a good applicant gets squeezed out by the competition.
Equally I think PPL is a fairly small course so I'd imagine it would be hard to draw any statistical inference from year on year intakes as I think most colleges will only have 1-3 students being taken on in a given year. So if a college takes on one more or one fewer student in a given year, that would throw the averages out considerably...
As above I think it's not something to worry about and you should just focus on finding one you generally like the feel of - bearing in mind you may well end up at another college in the end anyway!