HDD may go to standby mode a bit later than the time you specified when data write is in process. This is related to the data writing behaviour of the embedded Linux system.
When you save data to the NAS, the data is written to the system cache first and then to the hard drive(s) on the NAS. The write time depends on the size of the data. After the data is written and there is no access to the hard drive, the hard drive(s) go to standby mode according to your settings.