Based on the other's answers, it's obviously complicated (ha!) but my understanding is that it's a combination of popularity AND location. The numbers do correspond with the map on the side of the page, and when you go to another page of sitters, the numbers start over from 1-10 again.
Rover doesn't disclose its "ranking" algorithm, but I think it's a combination of reviews, response rate, completeness of profile, how they compare to other sitters in the area, etc. Sitters know how Rover measures some of this information, but not all of it. It also seems that Rover will provide a boost towards to top for new sitters in order to increase their exposure, at least for a period of time.
So no one but Rover employees know how to fully answer your question Michelle! I would take their ranking order into consideration, but with a grain of salt. Read the profiles and reviews to determine who is the best fit for you and your dog!