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

An employee of a business could indeed be considered, and labeled, a manager despite the absence of subordinate employees working under the individual in question. The determining factor usually involves the nature of the tasks for which the employee is responsible. Not all management-level responsibilities require multiple personnel. If an individual is trusted with important responsibilities, such as over corporate strategy, operation of expensive and essential machinery, or documentation required by government agencies, such as...

An employee of a business could indeed be considered, and labeled, a manager despite the absence of subordinate employees working under the individual in question. The determining factor usually involves the nature of the tasks for which the employee is responsible. Not all management-level responsibilities require multiple personnel. If an individual is trusted with important responsibilities, such as over corporate strategy, operation of expensive and essential machinery, or documentation required by government agencies, such as for environmental protection, he or she could be given the title of manager. Whether that individual has staff below him or her is immaterial. In the world of government relations/lobbying, for instance, many employees of large corporations dependent upon government contracts are assigned to lobby both executive and legislative branches of government on behalf of their employer. These government relations specialists are routinely given the title of "manager" both in recognition of the importance of their work to the corporations that employ them and to give these employees the cachet necessary to secure meetings with important government officials, such as members of Congress and their staffs. The government relations specialists, however, do not have staffs, although they may share an administrative assistant. 

In conclusion, managers without subordinates are still managers because of the importance of their individual contributions and because of the necessity of providing such managers job titles commensurate with the levels of people with whom they must work.