More pictures on Mac than on iPhone
Why is there more pictures downloading to my Mac than there are pictures in my iPhone?
MacBook Air 13", macOS 10.15
Try Settings/Your Name/iCloud and slide off Photos syncing. Wait a few minutes and then slide syncing on. Allow time for syncing to occur. The Photos sync is very, very slow.
Last resort - have you tried signing out of iCloud and then sign back in?