Some possible feedback loops:
Well known universities have more funds available and will be able to attract the best teachers away from the lesser universities.
Researchers may be willing to cite work from well known universities over similar work done at lesser universities.
The work done at other universities may be in a language that the researchers are not familiar with.
And so on. Quality in a vacuum is a very precise term but in the real world you'd have to discount for all of those factors.
In the tech world we have a similar example: Silicon Valley is 'where it happens' because it has a head start that is impossible to compete with which automatically attracts new talent and funding to a degree that it starves locations elsewhere resulting in an equality that dwarfs any qualitative effects that you may want to measure.
