The reason for this difference lies in the way that hard drive manufacturers measure the storage space compared to devices.
To a hard disk manufacturer, one KB is 1000 bytes, one MB is 1000 KB, and one GB is 1000 MB.
Essentially, if a disk is advertised as 64GB, it contains 64* 1000 * 1000 * 1000 bytes. This equals 64,000,000,000 bytes of space.
Computers, however, do not read this in groups of 1000. They use groups of 1024. According to a computer, a KB is 1024 bytes, a MB is 1024 KB, and a GB is 1024 MB. To work back from the 64,000,000,000 bytes above:
(((64,000,000,000 / 1024 ) /1024 )/1024) = 59.6 GB
Windows has always calculated hard drives as powers of 1024 while hard drive manufacturers use powers of 1000. You will find this consistent with any hard drive, portable USB stick, or memory card you buy.