Why isn't Mac Safari syncing iCloud keychain passwords?

[iMore Question]

I just migrated my 2016 MacBook Pro to a brand new 27" iMac. iMac Safari appears to have migrated my saved passwords from my MBP's Safari (~80), which apparently did not include all of my iCloud keychain passwords as shown on my iPhone and iPad (~450). Software is current on all of the devices. The MBP was running 10.13.3. iCloud Keychain is on on all of the devices. Any ideas? Thanks!