iPad memory used if far higher than sum of all App's memory usage
When your iPad is connected to the computer, click on the Device and look at the bottom of the screen. There is a bar that shows how much data each category is using.  What is the size of the "other data" category?

Your iPad stores not only the Apps but there is a lot of stuff like:
Browser cache,
Mail cache,
Mail messages,
Mail attachments,
Safari's Reading List,
iTunes Backups,
Game data,
Saved data files,
Call history,
Notes,
Media,
Voice memos,
etc.

To see what is using the memory space on iPad: Settings->General->Usage

In this list: All installed Apps are listed, tap on any App to see the actual App size and the amount of documents and data used by this App.

To delete this data, you have to delete the App, restart your device (press and hold Home and Sleep until you see the Apple logo) and reinstall it from the AppStore.