Difference Between Capacity and Avaible on Ipod touch settings?

On my ipod touch I go to settings and then I see Capacity. 14.5 Gigs. (My ipod is 16 gig) and available says 9.0 gig. I was wondering why theres more then a 5 gig difference. Why am i missing 5 gigs? :O
4 answers 4