This is an eternal struggle with Windows.
gb sllacker is almost right (BTW, why resurrect such an old thread?). Basically HD manufacturers advertise in one measurement unit and Windows displays it in another.
HD advertisements come in Gigabytes or Terabytes which equal to 1,000,000,000 or 1,000,000,000,000 bytes respectively. That follows the meaning of the classic prefixes, a "kilo" means "one thousand", period. There is no two gigabyte or something like that, there is no "decimal" or "binary" gigabytes, such prefixes have a well defined meaning (multiples of 1000) and computers haven't changed that at all, just like kilometers, kilograms or anything else.
What Windows actually does is to use
Gibibytes in calculations. Those are the "binary" prefixes which use multiples of 1024 instead. Note that all those have slightly different prefixes (KiB/MiB/GiB,
NOT KB/MB/GB). Typical confusion comes from the fact that Windows does the math in GiB units, but displays GB everywhere. A long standing bug in Windows.
In fact, it should display 2TB=1.81TiB