Volume free space not working?

We’ve been using this for a while, but we’ve just noticed that the volume space doesn’t seem to raise warnings (just status:Normal) :
Here’s the verbose output.

Synology model: “RS3614xs+”
Synology s/n: “1620M9N181700”
DSM Version: “DSM 6.1-15152”
DSM update: Available
System Status: Normal
Temperature: 42 (Normal)
Power Status: Normal
System Fan Status: Normal
CPU Fan Status: Normal
Number of disks: 12
“Disk 1” (model: “ST6000VN0001-1SF17Z “) status:Initialized temperature:41
“Disk 2” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:42
“Disk 3” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:42
“Disk 4” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:40
“Disk 5” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:42
“Disk 6” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:42
“Disk 7” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:42
“Disk 8” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:41
“Disk 9” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:41
“Disk 10” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:42
“Disk 11” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:42
“Disk 12” (model: “ST6000VN0001-1SF17Z “) status:Normal temperature:40
Number of RAID volume: 2
“Volume 1” status:Normal
“Disk Group 1” status:Normal

WARNING – Synology “RS3614xs+” (s/n: “1620M9N181700”, “DSM 6.1-15152”), DSM update available

According to the web interface of the device, volume on is 93% full. Any ideas?