Some employees are called managers without subordinates. Are they managers and why?

There are situations in which an employee without subordinate employees might be considered a manager.  Bear in mind that human resources are only one aspect of management. Managers are called upon to manage time, materials, information, and/ or money. If they have control over and responsibility for these resources, it is reasonable to call them managers. A finance department might have just one employee, who is the Chief Financial Officer. He or she is often considered a management employee because he or she is there to manage the money. A manager might be in charge of a production facility that "employs" only robots working at various machines. In spite of having no employees to control, this person manages the material resources for production, as well as being responsible for managing at least one technological resource: the robots.  This manager is also managing time, since the production manager is the one scheduling production. There might be just one person in the public relations office of a smaller company. That person is managing information, certainly outgoing information, and likely incoming information, too.  Whether companies choose to designate such people as managers is up to them. There is no management god in the sky who dictates this, but you can see there is plenty to manage in a company, not just people.