I’m not a mac user, though it says something about ~160GiB beeing purgeable.
How does it usually account for backups?
Also it seems as if APFS supports snapshots, how are those accounted?
As a ZFS user who uses snapshotting a lot in automated ways, I usually do not see the snapshots in a filemanager, but only by using the specialised ZFS tools.
Lets say du -sh ~ tells me that ~ were 230 GB, df -h ~ only says 209G due to compression, while a zfs list tells me this:
$ zfs list -o name,used,available,referenced,compressratio rpool/safe/home/nmelzer
NAME USED AVAIL REFER RATIO
rpool/safe/home/nmelzer 302G 72.5G 209G 1.18x
As you can see, this subvolume refers to 209 G directly, while it has an overall size of 302G. The additional ~80G come from differences in the snapshots I keep for that volume.
Perhaps something similar is going on for your APFS?