Comparisons & alternatives · macOS's own storage tools
Why does Storage Studio's Storage Inspector show different numbers from the macOS storage bar?
Short answer: macOS groups storage into broad categories; the Storage Inspector measures folders directly and says what it couldn't read. Space still held by your Mac's own snapshots is also shown separately, which explains most gaps.
They measure in different ways, so small differences are normal. The macOS storage bar groups everything into broad categories and folds a lot into "System Data". The Storage Inspector walks your folders and reports what it actually measured, folder by folder — and when parts of the disk are off-limits to every app, its footer says so rather than guessing.
The other common reason is held space. After you delete or move something, your Mac's own recent snapshots can keep that space taken for a while. Storage Studio shows this separately as Pinned (or "held for now"), while some macOS views count it as free or purgeable.
If you want the space back now, the Local Snapshots manager can release snapshots that are already backed up. More in What's Using Space.