The ground changes as you cross the county line
Williamson County sits on shallow limestone, so post holes hit rock at depths that would be easy digging elsewhere. Much of Rutherford County has heavy clay that swells and shrinks with the season. Wilson County mixes both, and Davidson County adds tight urban lots, alleys and older property lines that rarely match where the grass stops.
Those differences change post depth, equipment, install time and price. Quoting a Nashville infill lot the same way as a five acre Lebanon property is how contractors end up in change order arguments.

