GBs of storage space unaccounted for on iPad 1

so, looking through ipad (ipad 1, 64 gb, wifi/3g), when shocked see had 28.7 gb of storage used.

looked @ each app using , added find added under 8 gbs.

have idea phantom 20 gb of space usage coming from?

have 5 books in ibooks, , none of them picture-books.
have 13 photos saved it.

can't life of me figure out space has gone...

 

what itunes say? (referring graph shows storage usage)

 

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