It comes on by default, if you turned it off a while ago only thing I can assume is the core machine learning services that are doing Apple Intelligences (gen AI) job might be using the same storage category. Just spit balling ideas though, but Apple is vague when it comes to system data.
Also Apple File System (APFS) is 'copy on write' and uses snapshots. Storage usage and availability is a calculation from tracked metadata not reality. Data isn't removed when you delete something, the OS removes the tag and system maintenance will remove it during garbage collection. However it won't remove deleted files if they are still referenced in the local snapshot.