Have you tried rebooting the iPad and then sync again? Or just check again after you reboot?
Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.
I finally figured out what was going on: Photo Stream was set to on, somewhere/somehow that added up to 621 images!
Once unchecked, the image count went down to 0 - and after about ten attempts to re-sync my iPhoto library (each time with an error message of varied number of unsynced files), I finally have ALL images back on the iPad.