Why does my iPhone have too many photos in a smart album after syncing from my Mac?

I set up a smart album on my Mac. It only contains the ~500 photos I expect. I selected that smart album to sync onto my iPhone. After the sync completes, that smart album on the iPhone contains approximately all the photos in my collection! Is there a way to fix this?

I don't believe you can sync photos from a Smart Album to an iPhone or iPad if you're using iCloud Photos on those devices.

You need to first turn off iCloud Photos on the iPhone, wait until all your photos are removed from the device, then try to sync the photos in the Smart Album via USB to your Mac.

If you turn iCloud Photos on for the iPhone, it will remove the photos from the Smart Album sync and replace them with the iCloud Photos images. That's the situation I think you're seeing now. There's no way around this other than to turn off iCloud Photos on the iPhone.