Can't get old pics off iPhone

I have an iPhone 6, and for some reason when I connect to iTunes, I see that hundreds of really old photos from iCloud are back on my phone. I'm trying to keep all pics in the cloud, but every other day I find there are hundreds on my phone. In Photos, it asks if I want to download all new pictures, but 99% of the pictures in question are already on my iMac. I have no idea what's causing this. I have my phone set up for optimizing photo storage, and have plenty of room in iCloud.