The duplicate detection is running on each device locally. It does not sync between the devices. Each device is running the duplicate detection over night, when you are not using it, and when it is fully charged. Your Mac may not yet have finished the scan for duplicates. It takes a very long time, because Photos is looking for similar photos, not just exact duplicates, so it needs to compare what the photos are showing, not just the file attributes. Your Mac is showing the detected duplicates on the go, is showing the detected duplicates it has already found while it is searching for more. You may want to start removing duplicates that are already shown. Over time the Mac will show you more.