mount – Why are certain mounted volumes not listed by diskutil?

mount and df show that disk1s5 is a mounted volume:

bash-3.2$ mount
/dev/disk1s4s1 on / (apfs, sealed, local, read-only, journaled)
devfs on /dev (devfs, local, nobrowse)
/dev/disk1s2 on /System/Volumes/Preboot (apfs, local, journaled, nobrowse)
/dev/disk1s6 on /System/Volumes/VM (apfs, local, noexec, journaled, noatime, nobrowse)
/dev/disk1s5 on /System/Volumes/Update (apfs, local, journaled, nobrowse)
/dev/disk1s1 on /System/Volumes/Data (apfs, local, journaled, nobrowse)
map auto_home on /System/Volumes/Data/home (autofs, automounted, nobrowse)
bash-3.2$ df -h
Filesystem       Size   Used  Avail Capacity iused      ifree %iused  Mounted on
/dev/disk1s4s1  233Gi  8.5Gi  149Gi     6%  356049 1561586480    0%   /
devfs           209Ki  209Ki    0Bi   100%     722          0  100%   /dev
/dev/disk1s2    233Gi  1.7Gi  149Gi     2%    2019 1561586480    0%   /System/Volumes/Preboot
/dev/disk1s6    233Gi  4.0Gi  149Gi     3%       4 1561586480    0%   /System/Volumes/VM
/dev/disk1s5    233Gi  3.8Mi  149Gi     1%      21 1561586480    0%   /System/Volumes/Update
/dev/disk1s1    233Gi   69Gi  149Gi    32%  594098 1561586480    0%   /System/Volumes/Data
map auto_home     0Bi    0Bi    0Bi   100%       0          0  100%   /System/Volumes/Data/home

However, disk1s5 does not appear in diskutil list:

bash-3.2$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         250.7 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.7 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD - Data     74.2 GB    disk1s1
   2:                APFS Volume Preboot                 1.9 GB     disk1s2
   3:                APFS Volume Recovery                1.2 GB     disk1s3
   4:                APFS Volume Macintosh HD            9.2 GB     disk1s4
   5:              APFS Snapshot com.apple.os.update-... 9.2 GB     disk1s4s1
   6:                APFS Volume VM                      4.3 GB     disk1s6

diskutil info does show information about disk1s5:

bash-3.2$ diskutil info disk1s5
   Device Identifier:         disk1s5
   Device Node:               /dev/disk1s5
   Whole:                     No
   Part of Whole:             disk1

   Volume Name:               Update
   Mounted:                   Yes
   Mount Point:               /System/Volumes/Update

   Partition Type:            41504653-0000-11AA-AA11-00306543ECAC
   File System Personality:   APFS
   Type (Bundle):             apfs
   Name (User Visible):       APFS
   Owners:                    Enabled

   OS Can Be Installed:       No
   Booter Disk:               disk1s2
   Recovery Disk:             disk1s3
   Media Type:                Generic
   Protocol:                  PCI-Express
   SMART Status:              Verified
   Volume UUID:               BE235512-940B-4A80-A1C7-7B6CB924E1D6
   Disk / Partition UUID:     BE235512-940B-4A80-A1C7-7B6CB924E1D6

   Disk Size:                 250.7 GB (250685575168 Bytes) (exactly 489620264 512-Byte-Units)
   Device Block Size:         4096 Bytes

   Volume Used Space:         4.0 MB (4034560 Bytes) (exactly 7880 512-Byte-Units)
   Container Total Space:     250.7 GB (250685575168 Bytes) (exactly 489620264 512-Byte-Units)
   Container Free Space:      159.9 GB (159881302016 Bytes) (exactly 312268168 512-Byte-Units)
   Allocation Block Size:     4096 Bytes

   Media OS Use Only:         Yes
   Media Read-Only:           No
   Volume Read-Only:          No

   Device Location:           Internal
   Removable Media:           Fixed

   Solid State:               Yes
   Hardware AES Support:      Yes

   This disk is an APFS Volume.  APFS Information:
   APFS Container:            disk1
   APFS Physical Store:       disk0s2
   Fusion Drive:              No
   Encrypted:                 No
   FileVault:                 No
   Sealed:                    No
   Locked:                    No

So why doesn’t diskutil list show it? I thought diskutil list would always show all volumes, but it seems that certain mounted volumes are hidden.