It relies very heavily on student satisfaction. As a result London Universities do badly. And perhaps Universities with high proportions of overseas students, or other specific student demographics, do badly.
Student satisfaction is an odd one. I am convinced that Bournemouth U bobs up and down depending on how much beach weather they had that summer.
Also what you think when you graduate may be very different to what you think ten years later. That degree from Imperial may well have opened a lot more door than Nottingham Trent. Even though it was very hard demanding and so not as much fun at the time.
LSE has shot up. For a long time it was a matter of pride amongst students that they came bottom of student satisfaction tables, and so they filled out questionnaires to ensure that happened. Proved how tough they were to stick it out. Perhaps league tables like this have the advantage of encouraging Universities to think more about the overall student experience.