Q: Why on iTunes does it say my iPad 2 capacity is 13.4 gb but I have the 16gb version

See the link below.

http://guides.macrumors.com/Hard_Drive_Size_Discrepancy

Some space is taken up by how it's formatted, the definition of a gig, and iOS and the built-in apps - iOS storage reporting.

The 16gb figure is the total amount of space, but the OS and preinstalled apps use some of that space already.

Your computer is the same way. A 1TB will show as having only about 940 GB of space and when you add an OS and apps, it's even less. As in the above link, HD manufactures and flash drive manufacurers report space differently then the OS on your device.