Different username WHY?

I’ve had an account for my son for years now under a certain username and gmail account. I just tried to put his account on our new Xbox one S upstairs after moving theirs downstairs. I used the same gmail account and password and after it added the account.
It has a completely different username but the nickname is still the same. The account is still on the Xbox one downstairs with the old username. Why is it different upstairs? All of his friends and achievements are gone. But the nickname is the only thing
that is the same. What is going on?

:)

 

halo.xbox.com

Someone has already taken the username of your Gamertag. You will just have to use a different username on that website.

 

Cheats and tired of it!

It isn't any different on Playstation Network (usernames and other rules).

 

It sounds like you accidently created a new account on the new console instead of adding the old one. Are you 100% positive you entered the correct email address associated with the account on the new console?

 

If you sign into the account downstairs and go to settings and under the account tab go to sign in security and passkey it will show you what email is associated with that account.