Without going too far , the Winner of the Club World Cup, everyone or most of them were going for PSG Because of the tremendous playing Machine they had, but what happened? PSGF lost against a Chelsea Team that was nowhere to be seen that they were going to Win , this is a clear Example of what they are Saying.
Seeing what PSG was doing with big teams starting from Champions League down to when the club world cup started, you wouldn't want to bet against them. Chelsea on the other hand was a bad team using lucky chances to advance further, if i can remember vividly, two of their matches did go into extra time before they were able to make up the opportunity. How Chelsea was able to defeat PSG is what makes football interesting and unpredictable.
Betting on the underdogs is most times necessary, for the engagement and the fun to it, wondering how those who bet on Chelsea to win against PSG must have felt after seeing 3 goals from Chelsea ahead of the favorites, this is a typical example of having fun in gambling.