I didn't. Facebook did. The register plugin is a Facebook plugin and it pre-fills your data if you are logged in to FB. You can clear up the form if you don't

I didn't. Facebook did. The register plugin is a Facebook plugin and it pre-fills your data if you are logged in to FB. You can clear up the form if you don't want to carry over your FB credentials. There is a little cross on the right to where your name is filled - click that and your fb credentials will be cleared.