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 ...

update:
after understanding the photos number calculation as Mr. RedEagle2000 explained down, I checked to import all photos in iTunes and hit Sync, iTunes began to copy photos to my iPhone as iTunes top bar was loading, it took about 7 minutes to copy all of them, once finished I opened Photos app on my iPhone I got shocked when I found that only 4 photos have been imported from Mac under a newly created category: "Last Import: 4 from my mac" , restarted iPhone, unchecked Sync photos, re-sync again, then checked it back and synced again, but nothing changed, why is this happening ?