Why is the mount point 2 times smaller than the ZFS pool?

Question

FreeBSD

Subject.
I run in freenas
Created raidz2 a pool from 6 disks.
And for some reason, the real volume is almost 2 times less than it should be.

 /mnt# df -h
Filesystem                   Size    Used   Avail Capacity  Mounted on
freenas-boot/ROOT/default    9.6G    510M    9.1G     5%    /
devfs                        1.0k    1.0k      0B   100%    /dev
tmpfs                         32M    5.3M     26M    17%    /etc
tmpfs                        4.0M    8.0k      4M     0%    /mnt
tmpfs                        1.3G     96M    1.2G     7%    /var
freenas-boot/grub            9.1G    6.8M    9.1G     0%    /boot/grub
data                          22G    170k     22G     0%    /mnt/data
 /mnt# zpool list
NAME           SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
data          39.8G  1.23M  39.7G         -     0%     0%  1.00x  ONLINE  /mnt
freenas-boot  9.94G   520M  9.43G         -      -     5%  1.00x  ONLINE  -

Answer 1

And announce the result of the zfs list and zfs get all data / data commands.
Perhaps FreeNAS sets the quota immediately so that the file system does not suffer excruciating pain if there is less than a certain limit on the dataset space. ZFS is in trouble in such cases.