Question: Questionable purgeable space
I have an encrypted disk image I use for storing work-related files on my laptop. Out of the blue, the 'available space' jumped from ~100MB to 2.88GB. I looked at Disk Utility, and saw that 2.78GB was listed as 'purgeable'. When poking around online, I found a couple things:
1. According to Apple , such content 'appears when you've turned on 'Optimize Storage''. Files marked as purgeable can always be downloaded again on demand.
2. 'Purgeable' content is files that macOS can delete when space is needed on a drive.
I do not have iCloud storage turned on. I do not have any settings for 'Optimize Storage' turned on. I'm not running Time Machine . None of the files on my disk image could be recreated from other files.
So... why does macOS think that there are files it can delete at whim? Should I be worried that my irreplaceable  files could disappear if I copy a large file to the disk image?
 I never fixed my problems with Time Machine backing up an APFS file system drive over the network to a shared drive, and got sick of the endless messages about having to start over from scratch.
 But not necessarily valuable... they are archives of past activity.
MacBookPro mid-2012-OTHER, macOS High Sierra (10.13.4), 1TB Samsung SSD