As a comment. A PIN is a local method for sign-in and means your account does not have an online method to sign-in. I assume you are using the Authenticator app (on your phone, or a second email address) to verify it is you if you need to sign-in to your account.
When you use your PIN on your computer the computer is registered and known by the system so allows sign-in. If another user tried to use their computer to sign-in it would be challenged and if they cannot provide the correct details then access is denied.