Functioning of Palo Alto Networks “URL Filtering” Censor System

The University of Fribourg (Switzerland) is censoring its Internet in very different categories, including "extremism" and "adult".

It was found out that sites like www.martinlutherking.org are flagged as "extremism" or that sites like www.fahrraddoktor.ch (DE) and LGBT party sites like www.360fever.ch (FR) are flagged as "adult", which are clear false positives.

See this article (DE) on it:

https://www.watson.ch/!917791439

It seems there's some keyword heuristics going on for that to happen.

Does someone know more about the inner workings of the system, or: how can this happen?