When I filter a list by department, why are other departments automatically selected?

Other departments are automatically selected via the Users drop-down because users can be associated with more than one department. Automatically selected departments are indicated by one of the following:

A minus sign (-): At least one user in the department also exists within the department that you manually selected.
A check mark (): All of the users in the department also exist within the department that you manually selected.

Example: If Bob's primary department is St. Louis but he is also a member of the Boston department, then when you select St. Louis, a minus sign (or a check mark) will appear to the left of the Boston department.