Unexplained difference in photo count in iCloud, iPad storage, and Photo.app iCloud says I have just over 600 photos. On the iPad's storage it says there are just under 300.

However when I go into Apple's Photos.app apparently I have 1800.

I count 900 though. Why are these numbers different and where are the other 900?
Check www.icloud.com and make sure that the 1800 amount is in the Photos section. If so then your iPad may have "optimize storage" turned on. 

If the photos arent on there then you need check and see which devices you have and the full amount of photos on them all: Mac, iPhone, iPad &amp; iPod.

Lastly if neither of these situations helped, then contact Apple Support and let them know that there is an inconsistency with whats on your iPad and your Mac vs the iCloud. They may refresh your iCloud account.