How is usage on volume on disk group calculated? And why does it differ from `du`?

I own a NAS542, it has 4 bays with 4 HDD. I've created an Disk Group with those 4 drives and it's running with ext4 in raid 5. The HDD have 5.46 TB usable storage each.

When I go to my storage manager I can see that 58.92% (9.35/15.87 TB) of my disk group is in use.

However, when I login using SSH and run `du` on the /i-data/sysvol/ folder I get different numbers. If I add these numbers together I get somewhere around 3.5T.

/ # du /i-data/sysvol/* -sh

1.5T    /i-data/sysvol/Joop
1.1M    /i-data/sysvol/admin
12.0K    /i-data/sysvol/aquota.user
685.6G    /i-data/sysvol/backup
12.0K    /i-data/sysvol/document
70.0G    /i-data/sysvol/lost+found
225.8M    /i-data/sysvol/music
28.0K    /i-data/sysvol/photo
74.4G    /i-data/sysvol/public
16.0K    /i-data/sysvol/raspberry
854.9G    /i-data/sysvol/server
133.7G    /i-data/sysvol/software
104.7G    /i-data/sysvol/video

How is the volume usage in the Zyxel web interface calculated?

What is the volume usage in the Zyxel web interface including that isn't in the `sysvol` folder?

How can these numbers differ so much?