You buy a drive labeled 1 TB, plug it in, and your operating system reports 931 GB available. Nothing is missing and nothing is broken -- two different, both technically correct, definitions of a gigabyte are being used, and they don't agree with each other.
Decimal vs. binary units
Storage manufacturers use the decimal (SI) definition: 1 KB = 1,000 bytes, 1 GB = 1,000,000,000 bytes -- clean powers of ten, consistent with every other metric unit. Most operating systems, however, historically calculate and display storage using binary units under the same familiar names: 1 KB (really 1 KiB) = 1,024 bytes, 1 GB (really 1 GiB) = 1,073,741,824 bytes -- powers of two, because computers work in binary.
Why the gap grows at larger sizes
The ratio between 1,000 and 1,024 compounds at each unit level. At the kilobyte level the gap is small (about 2.4%). By the gigabyte level it's roughly 7%. By terabyte, it's about 9-10% -- which is why a 1 TB (decimal) drive shows up as roughly 931 GiB (binary) in your file explorer, and the gap looks larger the bigger the drive gets.
Which one you're actually looking at
Officially, KiB/MiB/GiB (kibibyte, mebibyte, gibibyte) are the correct binary unit names, while KB/MB/GB should technically mean the decimal versions -- but in practice, most software still labels binary values as 'KB/MB/GB' out of habit, so the label alone often doesn't tell you which system is in use. When precision matters (server storage planning, matching an advertised spec exactly), the storage converters here let you check both interpretations directly.