Confirm the selected root and keep its disk mounted. Grant folder access and enable Full Disk Access only when protected locations are part of the intended scope.
Online-only cloud files, read errors, package internals, links, snapshots, purgeable space, and files changing during the scan can make totals differ from Finder or System Settings.
Quit heavy file-writing apps and run a new analysis. Compare the same volume, scope, unit, and time.
If one branch is missing, scan that folder directly. Do not infer that inaccessible space is safe to remove.