When your sync your Apple devices over iCloud and share a common keychain, iOS and macOS share their accounts over every device. For example I enter an account on a macOS (and the for the certain device generated login credentials), it prompts on another device with macOS and is there in the system too. Problem is, when I delete the account from the system where the account automatically is transfered, it disappears from the origin system as well. So I guess, when it would be possible to define an account each Apple ID (or another entity, don't know) then accounts can share over every device you have in sync.