Depends on how you define the size of 1GB. It isn't the same unit.
Hard drive manucafturers define 1GB as 1,000MB, and each MB as 1,000KB
Everyone else in the known free world (your operating system) defines 1GB as 1,024MB, and 1MB as 1,024KB. This is a power of 2.
The 1000MB unit makes the drive 'bigger' than it is. Unfortunately its been going on for years, as it makes the harddrives more attractive. Your size is pretty much right. There are also space requirements for the partition tables and other filesystem overhead, but that isn't the main reason for the disparity.