It’s a confusion within marketing. From the Wikipedia:
"Facts
[ul]
[li]As of 2007, most consumer hard drives are defined by their gigabyte-range capacities. The true capacity is usually some number above or below the class designation. Although most manufacturers of hard disks and Flash disks define 1 gigabyte as 1,000,000,000 bytes, most computer operating systems calculate a gigabyte by dividing the bytes (whether it is disk capacity, file size, or system RAM) by 1,073,741,824. This distinction is a cause of confusion, especially for people from a non-technical background, as a hard disk with a manufacturer rated capacity of 40 gigabytes would have its capacity reported by the operating system as only 37.2 GB.[/li][/ul]
[ul]
[li]The human genome contains 0.791175 GB of data (the 3.1647×109base pairs[4] represented as 2-bits).[/li][/ul] Gigabytes in different products
[ul]
[li]A DVD-5 format disc is capable of storing 4.7 gigabytes, or roughly 4.38 gibibytes. A DVD-9 is capable of storing 8.5 gigabytes, or roughly 7.92 gibibytes.[/li][li]One gigabyte is roughly equal to 18 hours of (http://en.wikipedia.org/wiki/MP3) music (at 128 kbit/s).[/li][li]One gigabyte is roughly equivalent to 11 hours, 40 minutes of Flash video (at 450x370).[/li][li]Most 6th generation and all 7th generation game consoles have game discs that are around 1 GB or more: Dreamcast (1.1 GB), GameCubePlaystation 2, Xbox, Xbox 360, and Wii (8.5 GB), and Playstation 3 (50 GB) (1.5 GB),[/li][li]Dual-layer Blu-Ray Discs and dual-layer HD DVD discs can hold about 50 gigabytes and 30 gigabytes of data."[/li][/ul]