Maybe cheaters were disqualified and your ratings increased.

I guess the ratings are rolled back when some solutions get plag and contest get skipped for some participants and if they were better in rating than you then your rating improves .So, when they are awarded back delta might increase.

That's the whole point of rolling back ratings

I remember Mike writing somewhere that ratings are rolled back when he is removing people for cheating. Ratings were prematurely updated so that people can participate in the upcoming contests otherwise people who became CM from expert in the last Div 2 will lose rating after participating in an educational round. So to prevent from this, ratings are updated and then later rolled back to remove cheaters.

Hello dear Radin !

Roll back of the ratings is for identifying the cheaters, and after identifying the cheaters, they will be removed from the competition and your rank will improve and you will get more ratings. I hope it was useful!

The cheaters were moved and then your ratings increased :)
btw, the rating of one of my friend Pub decreased by 1 when ratings are rolled back. I just want to ask for the reason because he says that he didn't do something wrong before rolling back.

In my case, my rating was initially increased to 1209 but after the rating changes it went down to 1191 (a +23 increase in place of the +41 given earlier), any reason for this?