[bug?] backup_nice not really respected?

# grep backup_nice /usr/local/directadmin/conf/directadmin.conf 
backup_nice=10

# ps lax | grep /bin/tar

4   739 23638 27088  30  10  11300  1264 wait   SN   ?          0:00 sh -c /bin/nice -n 10 /bin/tar czfp /home/tmp/admin.27088/username/user.resellername.username.tar.gz  --exclude-from=/home/tmp/admin.27088/username/backup/non_readable_files.list  -C /home/tmp/admin.27088/username backup non_readable_files -C /home/username/ --exclude-from=/home/username//.backup_exclude_paths  domains > /dev/null
                         ^^

0   739 23639 23638  39  19  20296  1896 sync_p DN   ?          1:14 /bin/tar czfp /home/tmp/admin.27088/username/user.resellername.username.tar.gz --exclude-from=/home/tmp/admin.27088/username/backup/non_readable_files.list -C /home/tmp/admin.27088/username backup non_readable_files -C /home/username/ --exclude-from=/home/username//.backup_exclude_paths domains
                         ^^

$ nice --help
Usage: nice [OPTION] [COMMAND [ARG]...]
Run COMMAND with an adjusted niceness, which affects process scheduling.
With no COMMAND, print the current niceness.  Niceness values range from
-20 (most favorable to the process) to 19 (least favorable to the process).

Mandatory arguments to long options are mandatory for short options too.
  -n, --adjustment=N   add integer N to the niceness (default 10)
                      ^^^^^
      --help     display this help and exit
      --version  output version information and exit