New iPad - Account Data already known

In addition to that I‘m also asking myself how the Account data comes to the new device as I did not activate iCloud Keychain during the setup of the new iPad. I only added the Apple ID of my wife and turned on some basic iCloud services like photos etc.  🤔. 
That’s strange, especially because I thought the secret key will never leave the device!? 

Maybe an expert from 1PW can explain on technical level how the data are synced as the keychain data seems to be stored different as the apple password app data which holds safari credentials etc.