Question about mayorship

So, if the mayor of a venue is the person who has checked in on the largest number of days in the last 2 months...

How is it possible for someone who is ousted as mayor on a certain day able to retake the mayorship on the very same day? Say Person A has 20 checkins and is the mayor. Person B checks in on Friday to give them 20 checkin-days, and then again on Saturday for 21 checkin-days and ousts Player A.

By my reasoning, Player A couldn't possibly oust Player B until Sunday (checkin on Saturday for 21, then on Sunday for 22). But several times, I've become mayor, then been almost-immediately been ousted by the person I just ousted myself.

Just trying to understand how that can happen when it doesn't seem logically possible.