Google joins GitHub and email on the way in.
One button, both directions
It sits on the sign in and the sign up screen and does the same thing on each. If the Google address is new here, the account is created and you land in the console. If it is already yours, you sign in. No separate registration step, and no verification email, because Google has already confirmed the address.
One address, one way in
An address that already belongs to an account is refused rather than merged. Sign up with Google on an email you registered with a password and you will be told to use the password instead, so an account is never quietly forked in two.
You are matched on the Google account itself rather than on the address it currently carries. Change your work email at Google and you arrive in the same account, with the same team and the same applications.