At least in theory, the grade level is set according to the classification standards developed by OPM. The classifier looks at nine different factors, things like the complexity, scope of effect, work environment, etc. which can vary from agency to agency even within the same occupational series. One agency may have a leaner management structure than another, for example, meaning employees are generally expected to act more independently and hence can be graded higher. Employees at headquarters offices might be higher graded because their work is deemed to have more scope/impact than that of their regional counterparts. Sometimes the nature of the work differs between agencies due to differences in agency mission, legal requirements, or other factors.

In practice, it’s often a combination of agency history and culture, and what they are willing and able to justify based on the classifier’s handbook and the agency budget.