Backup size showed in urbackup is different from filesystem

I need help to figure out what is going on with my server.
I have a zfs pool:

#zpool list
NAME SIZE ALLOC FREE EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
backup 2,72T 609G 2,12T - 12% 21% 1.00x ONLINE -

#zfs list backup
NAME USED AVAIL REFER MOUNTPOINT
backup 405G 1,36T 405G /media/BACKUP

and using the cmd in /media/BACKUP:
#du --max-depth=1 -h .
512 ./urbackup_tmp_files
803M ./frevo
1,2G ./udinese
4,5K ./clients
356G ./athena
1,5K ./jazz
41G ./pop
6,5G ./biometria
637M ./urbackup
406G .

In urbackup web interface on server:

  1. the size of frevo computer is 391.15 GB.
  2. the size of pop computer is 121.05 GB
    the others is like the same as seen in zpool.
    This look very strange to me and can not figure out what is going on with my server.
    It’s a new install and only 1 full file backup of each client was made.
    Anyone can help me with this?