iPad showing wrong storage usage
I've run into this before. IOS hangs on to some files even though it should purge them. Often times it will also show up as the "other" part of usage in the space used graph being much bigger than it should. It's easy enough to fix though.


Back up your phone with iTunes. I would encrypt the backup so it saves your passwords.
Now we wipe the phone: settings > general > reset and erase all contents and settings.
Once that is done restore the IOS device. Plug it back into your computer and use iTunes to restore from the backup you just made.


Nine times out of ten that will get rid of all the cruft sitting around on your iPad (or iPhone). 

Why this is necessary... well ask Apple.