I recently had a project that involved processing a lot of digital photos. I had a number of folders temporarily on the desktop for various versions of the original set of unprocessed photos, and all together, this took up a nice little chunk of SSD space. Now that the project is completed, I deleted all of these folders. But because I’m a nerd, I wanted to watch the “used” space on the SSD decrease as I deleted everything, so I clicked “get info” for the boot drive and then “empty trash.”
Immediately, the used space decreased by the appropriate amount, but then I watched as the used space number slowly started climbing back up to near where it was before. The drive is still just over 50% occupied, so this isn’t a matter of much urgency, but it still seems strange to me that it did what it did. I like to keep a “clean house” so to speak so I run CleanMyMac regularly, and weed through other large document folders occasionally as well.
If you are using Time Machine for backups, this could be related to Time Machine. Time Machine keeps local snapshots until it can copy them to an external drive or a NAS, and these, of course, consume space. When you deleted the files, Time Machine could have kept them in the snapshot to include them in the next backup as that’s the last time it will “see” them.
Also, with APFS, which relies a lot on somewhat flexible space management, I would not worry about this too much as any temporary use of space (for Time Machine, or a swap file…) will eventually get released.
Exactly what @dario said, you can delete the TimeMachine & APFS snapshots, it’s fairly low risk however doesn’t really mean anything because this is free space that will be used if required, but is giving you a little bit more redundancy if things go wrong. Perhaps the golden rule if applicable here - if it isn’t broken, think carefully about fixing it.
(personally I’ve done this previously for exactly the same reason as you, not logical, but something about the way we are wired I guess).