Atalanta was not a weak opponent. However, Inter Milan players managed to perform very well against Atalanta. And Inter Milan won by a big margin of 4-0. The performance of the Inter Milan players was really very organized. So beating the Atalanta team was not a difficult task for Milan.
On the other hand, since Juventus managed to perform well in the first two matches, we also expected Juventus to continue their winning streak against Roma. But Juventus' performance against Roma was poor. I don't see a chance to say that Roma team was a strong opponent either. In those matches, Roma drew two and lost one. In this season, the Roma team has not yet won any match. So this group cannot be called strong. Juventus cannot play well in a home match against such a team. If Juventus want to win the title then the winning streak must continue. There is no alternative. If the performance is irregular, winning the Serie A title will not be possible for Juventus.
At the beginning of the season, Atalanta had not found their best performance and it was the opposite of Inter Milan so they could win the match easily.
I have noticed in Serie A since long ago that several top Serie A teams were not able to maintain consistent performance at the beginning of the season and that is also happening this season so we cannot measure the strength of the current team but what is certain is that if we look at the overall strength of the squad of several Serie A teams this season, it is indeed very likely that the competition for the Scudetto title this season will be much fiercer even though Inter Milan remains the favorite.
And Juventus should not be underestimated in my opinion, they made a lot of changes and this season is much better.