PullReview identifies the author by the email address mentioned in his/her commits. If that email address is known to GitHub, PullReview is able to make the link between you and the email. You can set up your emails via the GitHub settings page. Once done, just logout/login so that we can take that into account.
If you want to change the author's email mentioned in an old commit, it is possible to do it with a script. But this is a destructive action as it will rewrite all the repository history.