Incorrect user identification.
I created a new Hg projected at home and used the username tstone to check in some changes. Later, I uploaded the repository to bitbucket. When doing so, I had to create a user account (tstone2077). When I uploaded, it says that bitbucket's user tstone had contributed to the project.
The example of this can be found at http://bitbucket.org/tstone2077/svnautomaticgatekeeper./ tstone (my pre-bitbucket account) created the first 3 revisions.