Q: Why does my iPad not contain the storage capacity that it is marketed

The iPad lists the users storage capacity, it does subtract the GB used for the operating system. 

My 32 GB iPad, has 28.4 GB of user storage.

On your iPad, go to

Settings

General

Usage

'show all Apps'

This gives you the list so you know what is using all that storage space.

The operating system uses about  3.5 GB

The 57GB you observe already accounts for some space.  For example, iOS takes up space.  So does working storage.  Indeed, your device was built with 64GB.

This article may help explain things:

http://support.apple.com/kb/TS2419

This does subtract space. Like my iPod Touch 64GB, that has a capacity of 57.244 GB.