IPad Air storage is lying?

My iPad Air (it's updated to iOS 9.2.1) says i have used 26.5gb, and i have 0 bytes of storage left. I have added up the storage that each app i have takes up, and it adds up to like 5 or 6gb. I connected it to my laptop and it said that 17gb or something is being taken up by 'documents and data'. When i go into each app in the storage settings, it says that the documents and data only takes up a small amount of bytes.
Added (1). How do i clear the so called documents and data (the most i have for an app is 560kb)
Added (2). I have like 10 photos, none in recently deleted and my message conversations are deleted often
Added (3). its not 'other' data, it's purely 'documents and data'

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