First, like pickledbeans said, there is quite a lot lost to inode space for the filesystem.
Second, hard drive manufacturers are cheap and give your bytes to you in linear space, so 13Gb is:
Meanwhile every operating system on the planet treats 1k as
1024 bytes. (2 to the tenth) So 1 Meg:
1024x1024 = 1048576 bytes. (2 to the twenthieth) So 1 Gig:
1048576x1024 = 1073741824 bytes.
13000000000 / 1073741824 = 12.11Gb so really the drive probably has more linear bytes on it than it claims (hard to make the platters so bloody precise)