I know for the maths subject group (inc. compsci) that everyone has interviews at at least two colleges, your first choice, and an allocated one (presumably an undersubscribed one). After each interview, the tutors write up a report which is added to your application details on the university wide database, along with GCSEs AS/A levels etc, maths test score. Your application is then given a ranking. Other colleges can look at the reports and pick out people who they want to interview. I got the impression that this system was used for quite a few subjects, but i may be wrong.
In theory, anyone could be picked out of the database, regardless of whether their first choice college was interested in them, too. (they'd fight over you if necessary, later on), but i was told that if the tutors can't take you but think you're worthy of a place somewhere, they'll put extra shine on your reports to make sure another college will have a look at you. This is what colleges mean when they say they are very sucessful at placing good candidates that they can't take elsewhere in the university.
In the recent proposals to reform the admissions system, making this automatic second interview system the norm, across all subjects was a second option, rather than centralising the whole process, and is the one that is most likely to be adopted, i suspect.