Not all matches can be played equally well. Sometimes you can underestimate your opponent, or when playing at another team’s stadium, or it just happens that you are unlucky in a certain match, but that happens rarely. Like it was in the PSG final against Chelsea, it was clear that in that match PSG either underestimated Chelsea or it was one of the reasons mentioned above, because nobody expected such a defeat from PSG. But Chelsea also cannot win every game. They are a strong team, but they also have different kinds of performances. I think they are capable of winning the Champions League, but I don’t think they can do it this season.
There are games (and this happens in football manager a lot) where one team has %70+ possession, 20 shots, 10 on goal, and then they lose to a team with 1 shots on goal that became a score. So it's obvious that luck plays a big role, and sometimes players are just not on their day.
PSG won every single thing they can since last year, aside from just CWC final, which would have been another 30-40 million at least, maybe more. That is a good tournament, but it gives already big teams, a lot of money, that increases the gap with other teams. So those teams needs to utilize it, and find a way to get in, even if it's just some method that would be lowly. I think it's clear that we are going to see some smaller teams get a lot bigger.