For a 64GB drive, there's 4,719,476,736 bytes difference between gigabytes as described by drive manufacturers and as described by everyone else. Drive manufactures use a divisor of 1,000 instead of 1,024, thereby inflating the advertised size of the drive. Formatting then takes up a certain amount of space (think of it a bit like putting dividers in a filing cabinet - in order to organize it, a small amount of space is consumed). How much formatting consumes varies according to the file system and sector size used.
(Note that the divisor is used three times to create GB from bytes.)