It really isn't that weird. Marketing departments advertise drives as a certain size even when the real size is smaller. For example,
1,024 Bytes = 1 Kilobyte (KB)
1,024 Kilobytes = 1 Megabyte (MB) (1,024 * 1,024 = 1,048,576 bytes)
1,024 Megabytes = 1 Gigabyte (GB) (1,024 * 1,024 * 1,024...