Decimal and binary units
Storage marketing usually uses decimal units: 1 KB is 1,000 bytes and 1 GB is 1,000,000,000 bytes. Operating systems may show binary units such as KiB, MiB, GiB, TiB, and PiB, which use powers of 1024.
Why the number can look different
A drive sold as 1 TB may appear smaller in some operating-system views because decimal and binary units are being compared. The capacity did not disappear; the unit changed.