Few things to highlight:
1. The match held in Gewiss Stadium (Atalanta's home).
2. Atalanta played with their best lineup.
3. They didn't receive any red card.
This is a very bad result for Atalanta, this was supposed to be a sure win for them. Pisa wasn't good at all, Atalanta isn't able to maximize the chances they created.
I don't know Pisa very well. This is the first time I am watched team Pisa's match live. Although Atalanta's overall performance was good, their result was not good. It's a bad result in a home match, sharing points with a team like Pisa. But Atalanta's players were able to create a lot of scoring opportunities. Atalanta managed to have 21 shots with 66% ball possession. But even then the team couldn't score more than one goal. I would say Atalanta was unlucky. Often we see big teams fail to get full points against underdog teams. I would like to consider this an accident.