Freeing up space · What's Using Space (Storage Inspector)
Why does What changed say there's no earlier reading to compare against?
Short answer: It's the first measurement, so there's nothing to compare with yet. Storage Studio says so instead of showing misleading zeroes; check again later and the section fills in.
Because this is the first measurement, so there's nothing to compare it with yet. Rather than show zeroes — which would look like nothing changed — Storage Studio says plainly that it has no earlier reading.
Open the window again later, or press Measure again after some time has passed, and the What changed section fills in with what grew, what's pinned and what was reclaimed over that window.