IF you are using iCloud Photo Sharing (storing your photos in iCloud to make them available on multiple devices), then pay attention to the following.
There are issues with using iCloud photo sharing, the main one being that uploading images to iCloud can be very slow no matter your ISP speed is. The slow speed is caused by Apple’s server. Apple throttles the speed down to try to accommodate the millions of users uploading many millions of images all at the same time. The end result is that changes made on one device may not show up on another device even in the same week.
Related to that, to know what is actually in iCloud.com, you need to open a browser and log in to icloud.com using the “same” Apple ID that you are using on your devices (for example, I have a main Apple ID and several aliases. I need to log in using the main ID). And it also helps to be logged in with that same ID on all your devices. So, when looking at your images in iCloud.com, you may see duplicates, or missing images, or images there that aren’t on one or more of your devices. There are several factors involved with this discrepancy. Wait a couple of days for the discrepancy to clear up. It is about a duplicate image (in iCloud.com or one of your devices), it may be a real duplicate caused by the synchronization process. The recommendation is to log in to icloud.com and delete the image to the right as that one is most likely not the original. If the duplicate is on one of your devices, again just delete the one on the right.
There is no way on your iOS device (using Photos in iOS) or in icloud.com to see the size information of an image. All you can go by is the date, and that will most likely be the modified date and not the creation date. So choosing to delete a duplicate in icloud.com or on iOS is a crap shoot - but choosing the one on the right is most likely the best bet.
Finally, if the total number of images and videos is different on all your devices (like it is on mine), then wait - and wait - and wait - until the numbers match up, which they eventually should. And it may take many days for the match to happen. During that time, you may be adding/deleting/editing images on one or more devices, and that will certainly keep the numbers mismatched at any point in time.
Hence, if things don’t seem to make sense regarding the number of images and what image is on one device and not another, be patient and have faith that eventually some, if not all, discrepancies will clear up.
John R. Carter, Sr.