This surprised many people because of Milan's opening match result, having suffered a defeat against Cremonese. After that, Milan performed well. Honestly, I was also surprised that Milan managed to take a 2-goal lead in the first half against Napoli. Even after playing with 10 men on the field, they managed to maintain their lead despite so much pressure from the Napoli players. It was a victory from hard work, and I think Milan must have celebrated the win from that match.
Milan successfully bring down Napoli to second place, as we can see they're successful after they sign quality midfielders, both Modric and Rabiot are experienced players and they make their strategy works. Allegri know how Italian players are playing, so he use 5 midfielders the same like Inter's play.
Slow pace play style, that's what most Italian teams use.