If you only look at the settings on your iPad, then maybe some of the apps use more space.
As an example:
Did you take a look at your Mail app? When you open an attachment, it needs some of the iPad's usage. It's good to delete the account from time to time, to clear up a little bit of space. When you set if up afterwards, you should notice that it's usage is lower than before.
You could try restarting and resetting your device as well.
Restart: turn off your iPad, turn it on again.
Reset: press and hold Home and Power button until the Apple logo appears, release both and let the iPad power up.
In iTunes (computer) you might find another kind of usage: Other. Restoring is a good way to get rid of this.