I trusted that site with Barca being league winners for 20/21.
Never again.
It's prob the best site available for prediction, and you should reconsider if you want to be serious about it.
Thing is, such sites are not fortune-tellers.
They can't predict if key players will get injured or run into a terrible patch of form, and dynamics will get dismantled.
NO ONE can tell that!
What they do is they use adaptive models, with every game in there is more data to add up to the given one, and they can predict really well if you eliminate the outside 'noise' from factors not dependent on football itself.
The fact that Barca was predicted as league winners for 20/21 is that we were far better than our opponents in xG balance and all the metrics indicating the strength of a team in getting results.
The fact that it didn't work out at the end, has nothing to do with footballing reasons