For those with issues of incorrect storage in iPad, erase contents AFTER doing a backup, then do a restore. I gained 20 G. My photos were showing 12.8 G and went to 3.0, which is correct. I have been trying to correct this for two weeks and finally caved for erasing and restoring. It does take ah hour or two for complete restore all apps, photos, data, etc. but worth the effort.
iPad Mini, iOS 9.3.5, Storage issues