Google is our friend from wiki
"Although most manufacturers of hard disk drives and flash-memory disk devices define 1 gigabyte as 1000000000bytes, software like
Microsoft Windows reports size in gigabytes by dividing the total capacity in bytes by 1073741824, while still reporting the result with the symbol "GB". This practice is a cause of confusion, as a hard disk with a manufacturer-rated capacity of 400 gigabytes might be reported by the operating system as only "372 GB", for instance."
Link
http://en.wikipedia.org/wiki/Gigabyte
As well as some of the memory is used for the OS and so forth... (some times up to 1gb can be used for it...)| and 8gig ipod or flash will be reported to have 7.4Gb of space 6.8Gb sounds right as some of it has to be taken up by the OS...
This applies to any electronic storage except RAM.