I really think it is related to shared Apple IDs. I know the author states that they do not share one, but every time I've seen this happen, it has either been due to a currently shared Apple ID or one that has been shared in the past and logged into in just one place while the rest of the phone has been switched over to the new Apple ID.
Managing Apple devices is a royal PITA. Trying to change from 2-step to 2-factor authentication was not simple because I had a legacy 3gs device. I thought managing devices attached to my account via appleid.apple.com was enough. Nope. After much investigation, I learned there was also something buried via iTunes software. I am not sure where in the back-end the iTunes device list was being stored, but there was no connection between the iTunes device list and what was listed via the website.
The appleid.apple.com and iTunes device lists are also segregated from the Apple support website that has its own device list. Which appears to randomly delete stuff from my profile. With the recent revamping of their support website I checked it out, and they now only show 1 current Apple device. Not the previously listed 5 I actually have.
I don't know if I've actually found all the possible editable locations Apple device information could be stored with Apple.
