Is your iPod touch a 3rd or 4th generation iPod touch? On my 5th generation iPod touch the Capacity is 57.4 GB, Available is 38.0 GB and Used 19.4 GB. The flash drive or solid state drive has been formatted from the advertised capacity and less than a gig is used for the iOS. Hope this kind of helps you.
The difference between the advertised capacity (6, 16, 32 or 64 GB) and available capacity is taken up by the iOS and of how GB s are advertised and used. See:
An 8 GB iPod has about 6.4 GB available to the user.