Because anyway, Atalanta was also not weak at all at that moment, because Liverpool was also defeated by Atalanta. So yes, I think results like this can indeed happen because after all, in football we all realize that any result can happen, and like this is the end.
Atalanta was not a weak side but Bayer Leverkusen was the favorite in that match. They won a Liverpool that is not also in good form. Almost all Europa fans believed that the Italian side had little chance of beating Bayer. To be sincere, the outcome of the match was a surprise to many sports analysts and fans. That's the fun and intrigue of sports, unexpected happens all the time.
I never considered Atlanta as a weak team since they shock me by eliminating the whole Liverpool by defeating them at Anfield. I started to rate them high since then, and after seeing their victory over Marseille in the semifinals, I knew it will not be easy for Leverkusen in the finals, but Leverkusen on the hand didn’t look serious as they have seen how Atalanta lost to Juventus in the Copa Italia finals and how they ended in the Seria A table; but Atalanta tried by given them a shocking defeat at the end which made me very happy, not just because of Leverkusen’s loss, but the result of underrating a team that have history.