A Gigabyte is one billion Bytes of data. Most hard disk drive manufacturers follow the metric conventions of Kilobyte, Megabyte, and Gigabytes for specifying the drive capacity on their label and specifications (for example, 1000 B =1 KB, 1000 KB=1 MB and 1000 MB=1 GB) instead of the 1024 convention.
For example, given a manufacturer-specified 18.4 GB hard disk, the actual capacity reported by the disk is 17930455 KB = 17930455 *1024 = 18360785920 Bytes. This, if converted to metric convention, results in the specified 18.36 GB capacity rounded up. Thus, the actual capacity of the 18.4GB drive is displayed as 17.1 GB.