Cutting and pasting photos from phone to PC doesn't free up as much space as seems appropriate

iPhone 8 - this has happened while running various versions of iOS

My phone storage was almost full (62.8GB used of 64). I hooked it up to my PC and went into the DCIM folder and started cutting and pasting photos from the phone onto the computer. When all was said and done the folder on my PC was 8.11GB so it should have freed up 8GB on the phone but it only freed up 3.3GB. What the heck?

Side note: This is the typical method I use for "backing up" photos and videos and I have often suspected that the amount removed does not match the amount freed up on the phone but I generally fail to compare the numbers before I start cutting and pasting. My husband uses the same method for backing up photos and videos on his phone (iphone 6s+) and doesn't seem to have this issue.

Side note 2: I just went into iPhone Storage -> Messages -> Conversations. The largest one in there was the thread between the two of us which showed as 8.81GB. It was recently backed up so I deleted it. It dropped the used space from 59.5GB to 56.9. So this instance shouldn't have anything to do with copying to a PC. I deleted an 8.81GB thread and it freed up only 2.6GB. What's the deal?!