People Album and iCloud Photos

Hello,

I'm new to iCloud Photos, and was just wondering if someone could help me understand how the People album works in the iCloud Photos world. My understanding is that faces are now synced via iCloud photos; however, my Mac has finished scanning faces and the People album is ready, but my iPhone is still scanning faces with a long way left to go. If the faces are in fact synced via iCloud, why do both devices have to scan the photos when they are all the same photos and one device has already finished scanning all of them?

Thanks in advance for any insight you may have!