That doesn't really mean much on the long run. It is obvious that you can't just bet on the favourites every single game, because favourites do lose. If it was that simple, then we would all be super wealthy right now, how?
Well, when the leagues start, every weekend, bet on the favourites on all the leagues, anything that is under 1.70 for example, so it should be a "sure thing" and you would win, because favourites end up winning every game. But that doesn't happen does it? Favourites lose all the time, and this was just another example.
The finals is a different match altogether and no team should be underrated in a final game, for example in the Europa League final Manchester United were the favourites but Tottenham won by a goal, even though people claim they were lucky but a win is a win, similar to what happened in Chelsea vs PSG match at some point Chelsea had close to 6 odds with PSG about 1.6 but Chelsea suprisenly outsmarted them and won.
The league game is different from a final match and I agree that at the beginning of the league most big team in Europe starts well mostly when they play midtable teams they tend to do better but you can't compare that in a final match cause the pressure would be on both the underdogs and favorites, anyways Chelsea have been very lucky in finals they won the two Champions league against Bayern Munich and Manchester City as underdogs.