Iturralde said it best. Referees became referees because they love football most of them. If you love football, you must have a favorite team. It's logical, there's no such thing as 100% objectivity unless you are a robot made of mathematical algorithms and code. If you are not in Catalonia, that favorite team is most likely Real Madrid. The best refs, the most professional, are impartial. But some of the others, have these moments, where they just, maybe without even realizing it, sway it towards Madrid. I have zero doubts someone like Gil Manzano is a fan of Real Madrid for example. This may or may not show, depending on many factors and so on.
It's always been like that IMO. It's not something very obvious 90% of the time, but for sure it's real.