The Gunners are now slightly behind City in the Premier League table with a handful of games to go.
"It was a complete surprise," Wenger said on beIN Sports on how defensively they played against City recently.
You know that when you play for the championship at home with eight games to go and you have the opportunity to make a difference, you have to take it.
"The surprise of the weekend is that City have their nose in front again. Nobody expected that. Of course, one could have a bad result but not the two and that is surprising. Arsenal still have an advantage with their goal difference.
"So even if City have a draw and we win, we'd be in front of them. The frustration was a bit on the day (they drew at City). You didn't know after the game. OK it was a good point and a well-fought point because City didn't create much.
"I get as well the feeling that if they had pushed more, the three points were available there. You'll know at the end of the season, usually when you play against a big opponent and you can finish him off, you want to do it."