More than one system exists to define unit multiples based on the byte. Some systems are based on powers of 10, following the International System of Units (SI), which defines for example the prefix kilo as 1000 (103); other systems are based on powers of 2.
Your system calculates 1 terabyte as 1 tebibyte which is 2^40 bytes=1,099,511,627,776 bytes and the hardware manufacturers calculate 1 terabyte as 1 terabyte which is 10^12=1,000,000,000,000 bytes. That is where the discrepancy is.
in small print 2TiB*
From wikipedia:
Your system calculates 1 terabyte as 1 tebibyte which is 2^40 bytes=1,099,511,627,776 bytes and the hardware manufacturers calculate 1 terabyte as 1 terabyte which is 10^12=1,000,000,000,000 bytes. That is where the discrepancy is.