Why does my reported used and available disk space not equal the total reported via "df"? I'm running mac Big Sur (11.2.3).  I get some strange disk space readings when I query free space on the command line ...
s davea$ df -h /
Filesystem       Size   Used  Avail Capacity iused      ifree %iused  Mounted on
/dev/disk1s5s1  234Gi   14Gi   18Gi    45%  568975 2448556385    0%   /

I'm confused -- the size of the disk listed (234 GB) seems to exceed the used + available by quite a lot.  I've been noticing disk space issues even though there is 18GB free reported so I'm trying to track down what the problem is.
Edit: Per request, here's the diskutil output ...
$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI ⁨NO NAME⁩                 209.7 MB   disk0s1
   2:                 Apple_APFS ⁨Container disk1⁩         250.8 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.8 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume ⁨Macintosh HD - Data⁩     205.8 GB   disk1s1
   2:                APFS Volume ⁨Preboot⁩                 426.9 MB   disk1s2
   3:                APFS Volume ⁨Recovery⁩                613.9 MB   disk1s3
   4:                APFS Volume ⁨VM⁩                      6.4 GB     disk1s4
   5:                APFS Volume ⁨Macintosh HD⁩            15.1 GB    disk1s5
   6:              APFS Snapshot ⁨com.apple.os.update-...⁩ 15.1 GB    disk1s5s1
It's because on Big_Sur / is a read only system volume mounted on a snapshot. The size of this volume is around 15 go. The free size is 18 go, so the volume &quot;data&quot; may be around 200 go.
The command
diskutil list

Will give the real size of your system volumes.