Why my hard drives capacity appear smaller on TNAS?

[Samiii]

I have 4x 4TB Western Digital RED Hard Drives in this TerraMaster F4-210 4-Bay NAS running in RAID 5, where I would expect to have started with 12TB and the NAS is only seeing that I have 10.6 TB, missing 1.4 TB seems to be quit a bit, and maybe it has to do with the calculations of bytes counted in terrabytes, but it started the Calculations seeing 12TB during the set-up, and after the set-up, it shows I have (x) used of 10.61 TB.

[powerQ]

The hard drive capacity appears smaller than that claimed by the HDD manufacturer, because the TNAS file systems adopt the binary definition of storage units while HDD manufacturers adopt the decimal definition.

Decimal definition adopted by HDD manufacturers:
1 TB = 1000 GB; 1 GB = 1000 MB; 1 MB = 1000 KB; 1 KB=1000 Bytes

Binary definition adopted by file systems:
1 TB = 1024 GB; 1 GB = 1024 MB; 1 MB = 1024 KB; 1 KB=1024 Bytes

For example, if you buy a hard drive labeled as 8 TB in capacity, and install it on your TNAS, the actual size displayed will be as follow:
8 x 1000,000,000 Bytes / (1024 x 1024 x1024) = 7.448 TB