Como might be considered a weak club by you, but what they did to Napoli was the same thing they did to Milan, Fiorentina and Juventus. Como got all three points from these four strong clubs including Napoli, so I am not surprised. However, when you underestimate your opponents, you will end up been defeated. Napoli wasn't serious in this match, and it has cost them losing their position to Inter.
Juventus met Como twice this season, they both had there Italian first kick off together and Juventus won them 3:0 and the last time was just on this February Juventus still won them 2:1, so what I'm trying to say is that Como has not beaten Juventus yet but as for Fiorentina they did win them, another club they have not also won is AC Milan because they have also been a subject to them, so actually the yesterday was the only time they won their biggest team which is Napoli and it was there oversight that mad it so, they were too pompous to believe that they no more than Como.