Why doesn't Gnome Disk's Drive Setting work?

Gnome comes with "Disks". If you select a disk, click the three-dot button, and select "Drive Settings", it shows Standby or APM settings. But these does not seem to work. Judging by the power meter that I had attached the computer to, the disk did not seem to go sleep (spin down) even after setting the Standby to 5 minutes and APM to 127 (spin-down permitted) after a long inactivity. Then I executed hddparam -S. It made the disk sleep after a while judging by the reduced wattage on the power meter. Why doesn't Gnome Disk's sleep setting not work but only hddparam -S? Aren't they the same thing? Or am I misuse/misunderstanding Gnome Disk's settings?