Glossary
Purgeable space
Space the system may reclaim, which is not always free for a new file right now.
macOS can mark some data as purgeable: local snapshots and certain caches the system is willing to drop when an app needs room. Storage settings may count that capacity as available while a simpler free-space figure still treats it as used. The two numbers are allowed to disagree.
Disk Housekeeper shows a free-space readout in the menu bar. It does not redefine purgeable space, and it does not claim that a Smart Clean purges snapshots. If a clean's savings history and the Storage window tell different stories, check the Trash first, then read this term again.