How come my iPod (4th gen) says that I have used 6.0 gigabytes when really, I have only used 2.5?

When I press on settings, then usage- it has written on it that I only have half a gigabyte left, but when I calculated how much I was spending altogether, it only added up to 2 and a half gigs. I only have about 15 songs and 30 apps. I have a 8 gigabyte iPod and it is really frustrating me because I cant update my iPod to iOS 6.1 :( PLEASE HELP!