The Italian cannot find a reason why his team can play so badly one match and so well the next after the team claimed a 3-0 win over Nice on Sunday.
The Ligue 1 leader slumped to a penalty defeat in the Coupe de France against the minnows but bounced back in style, with goals from Jeremy Menez, Zlatan Ibrahimovic and Clement Chantome ensuring a handsome victory at Parc des Princes on Sunday.
"We saw a lot of intensity and aggression. It was tough tonight," Ancelotti told Canal +. "The key was there. I don't know why the team can play as well as they did tonight and as badly as they did against Evian.
"Anyway, it happened. We are happy with how we did tonight."
Meanwhile, PSG goalkeeper Salvatore Sirigu has stressed that his side is not invincible, despite the star talent within the squad.
"We're not untouchable," he added. "We showed a good attitude tonight. We wanted to show that Evian was just a blip. We wanated to bounce back, we did it well.
"We don't pick our matches. We don't always do as well as we want in terms of playing well or playing badly."
PSG holds a nine-point lead as the club chases a first Ligue 1 title in almost two decades.