Download before sharing to iCloud Album?

I’ve started using the iCloud Shared Album feature of Photos app and I am confused about something. I have the manage free space option in use, so I understand my phone uploads it’s images to iCloud then makes an iPhone version to keep on my phone to save space, leaving the full resolution version on iCloud. So... why, when I go to add photos to one of my shared albums on iCloud, do I have to wait for it to “download”? The full resolution image is already in iCloud. This is evidenced by the fact that your shared albums do not count against your space usage... so the albums just link to the coly of the image(s) already in your account. So again, why in the world would Apple need to download the full resolution image to my phone to add it to the shared album? It is doing this on my shared albums that I created on my account. Seems like a blatant misuse of bandwidth.