Google authentication links to user automatically

When you say you added google authentication, do you mean 2 factor author, or google login? Did you go to the account settings on that user and add google login?

Google login.

No, I did not add google login as the end user. I logged in from the login page for the first time using the google account.

The only thing the two accounts had in common is they both used the same email.

At no point did I ever link the two accounts.