I accidentally committed with the wrong author (Me <e@mail.com>), because I'd used that as a placeholder author, to protect my privacy. On the GitHub page, the author appeared as tCInUBkXfaMY, but they don't own the name Me.
To fix this, I amended the commit, backed up the master branch to another branch, then ran git push --force.
Even when deleting the backup branch, the contributor still appears on the project page.
The contributor appears nowhere on the graph (graphs/contributors), but still appears on the main page.
This only just happened, but I don't think it will fix itself - I saw the same thing happening on https://github.com/GandyT/GandyClient-2.0 (just a random repository I came across).