I try to use the Items "Free disk space on /" => vfs.fs.size[/,free]

When i launch a "df -lh" on the client, i have this result :

Code:
Sys. de fichiers Taille Uti. Disp. Uti% Monté sur
/dev/sda1 9,8G 5,7G 3,6G 62% /

When i launch this " zabbix_agentd -t vfs.fs.size[/,free]" on the client, i have this result :

3848830976 Bytes => 3.8GB

When i launch this on the supervision server "zabbix_get -s toto.server.fr -k vfs.fs.size[/,free]", i have this result :

3848830976 Bytes => 3.8GB

But on the dashboard (Last Data), i have :

17793036288 => 16GB !!

I use the default template linux.

The link on the forum with the screenshot => https://www.zabbix.com/forum/showthread.php?t=40798&page=1