Not only applied with football but also in other major sports which underdog teams are now beating heavy favorites, it's so happens that improvements are being recognize, in a gambler's perception it needed to adjust and not just being a loyal fans and believe with how your team will perform but instead trying to analyze deeper for better chance of recognizing decent odds and huge amount of possible profits.
That's why, when it comes to scoring goals, the weaker teams, or those we consider weak, can shine. And that's because football now has that, with players who believe in themselves and who can compete against the biggest teams and beat them. It's happened to the big teams, let's say a Real Madrid, Barcelona, a Bayern Munich, sometimes they fall to teams we don't imagine, and even more so in high-level competitions, like the Champions League.