Arsenal dropped 7 pts in the first 19 games and 12 pts in the next 12 games.
Form can change like that.
Impressively, 538 had them on fewer than 90 pts at the end of the season when they were on course to 100 pts...Maybe 538 underestimated City a bit, because such models abstract from important contextual factors, such as City having Pep, experience, ability to go on winning runs, etc.
Now, even if Arsenal regain their form from the first 19 games, they may find it difficult to finish 1st, unless they get a result at the Etihad and win the other games. It's not impossible. Football can surprise us, that's why it's a great game. It's not probable, though.
Edit: Arguably, the best case scenario for City is to win the title with Arsenal winning more pts than the Invincibles: 91-2 pts. I'd be very happy to win the title period, but it would be funny if they won more pts than the Invincibles and failed.