iTunes 12 doesn't show the correct number of photos to be synced and doesn't sync all photos I have iTunes 12.2 on a mac book pro with Yosemite, I use Photos app now instead of iPhotos after importing all of its events into iPhoto, plus I recently added around a few thousand pics and videos to the Photos app, the All Photos option in Photos app shows 4372 photos, whereas in iTunes when I select "copy photos from Photos" and "All photos and albums" options under the "Photos" tab it shows 3858 photos as the total number to be synced, where the hell those 500 photos have gone ?       

I searched for a solution but couldn't find any, I also checked out this question: Why is iTunes only syncing some of the pictures (seemingly randomly) in each Aperture Event to my iPad? but couldn't also help me ..

any help would be highly appreciated ...
I think that the 4,372 photos are all of your photos, as opposed to just photos to be synced. In other words, around 500 photos of those photos have already been synced to your iPad.