Is that the "total capacity" or the "free space"?
Total capacity is often seen to be "smaller" because a "GB" can be measured two ways: as 1,000,000,000 bytes or 1024 x 1024 x 1024 bytes. The drive manufacturers often quote the number that makes the drive look biggest.
Formatted capacity is smaller than total capacity because some of the space is used for indexes, directories, and other file system metadata.