The one displayed in the top right corner in GitHub is your GitHub username – this is the one you use to login to GitHub when you enter the site and when you commit over HTTPS, and the one that appears in the URLs of your GitHub repositories.
The one you create using
git config is your Git username – this is the one that appears as the author of your commits when you do
git log or
Since you can use Git outside of GitHub, those two usernames do not have to be the same.
Also, the GitHub username must be unique, but nothing forces uniqueness of Git usernames.