Freeing up space · Where your space went
Why did a space notice say the drop was caused by Storage Studio itself?
Short answer: Because one of Storage Studio's own jobs, such as a move or backup, was running and free space shifted. The notice says so instead of alarming you; Background Activity shows which job ran.
Because it was, and saying so is more useful than raising an alarm. While one of Storage Studio's own jobs is copying or tidying — a move to your fast drive, a backup, a clean-up — free space can shift for a while. The notice tells you it was Storage Studio's work rather than treating it as something wrong.
The space breakdown counts this properly too: work Storage Studio did is credited to it, not added to the unexplained remainder. To see exactly which job ran and when, open Background Activity from the More menu.