In Florida, and most southern states, the sheriff s department acts the same as the police department in those parts of the county that aren t in towns or cities. So although all of the sheriff s department car say Sheriff on them, most of the people in them are actually deputies and sheriff s department officers.This is opposed to the Northeastern states, where the sheriff s department has really specified duties (transferring prisoners, serving warrants, auctioning property to pay back taxes) and you always have to be in some sort of municipality.
They hire the county sheriff to police their city, this is not unusual.