I know here that here at least, they schedule people in the high-risk classes based on the time frame that the site is open. However, the vials usually contain a little more than the number of doses that is supposed to be in them. By combining residual vaccine, they can squeeze out several more doses. So rather than waste it, they grab anyone they can to come in at the end of the day to get vaccinated. In our case, they have complied a list of city, county, school employees, and other "essential" residents to call when they determine how many doses they are going to have left at the end of the day.