Within NaturalHR, managers can be defined in two ways. The first is a manager level user, this is anyone with the usergroup "Manager".
Managers are also defined by their job role, when creating or editing a job role you'll find the "manager level?" option, ticking this designates the job role as being manager level.
If you're viewing a report with a filter which matches this:
You might still find some employee level users appearing in the data which the report pulls through. This is because this field is only looking at the job role manager level field and does not check the usergroup of any of the users it pulls through to the report.