free space on ssd

The interesting thing about this is, that the size doesn't continually go down. It always gets say, to about 10 or 12 Gigs less than what I'll call normal & then suddenly (and without warning) it'll be back up to normal again. Sometimes it's a reboot that will rectify it or after several Windows updates, I'll clean it up and it will bring it back to the norm again.
Therefore, there's really no need to pursue this any further. I initially thought it might be something simple and it probably is, for example, the restore points. Although I would say, if it was a restore point, why would the reboot bring the size of the Drive back to where what I'll call a normal size, e.g. 150 plus Gigs.
I'll say thank you though for the input and efforts to clarify what's taking place. Much appreciated!

I feel I should add this to this ongoing and closed case saga. I noticed space went down from 153 to 148 Gigs. I deleted the Restore point although one I had named a bit back wasn't there. Whatever, I deleted what must have been automatically put there & upon reboot, I noticed I have 153 Gigs showing again according to Explorer and 153.6 according to Wiztree.
So obviously, these restore points must be created and if I do nothing, they eventually must be removed with new ones and thus the reason why I will suddenly see a low number, e.g. 148 back up 153. Weird and totally wonderful!

Since I have more than enough space on this Drive, I have something like 10 Gigs put aside for Restore points. I also have a System backup but have never used it to restore anything.
I also have the complete Drive backed up by AOMEIO Backupper Professional should something go awry and this laptop explode.