Itunes always re-syncs some existing photos & videos
For years, I have used Itunes to sync photos and videos. Typically, I will use it once every few months, with up to 100 new photos and a few videos. Yet when I sync to Ipad, it will variously sync 200-500 items, taking 30 minutes+, sometimes longer. Everything is up-to-date and properly managed on both PC and Ipad.
iPad, iOS 10