Usernames in git commit messages on Drupal.org
The usernames are the usernames for the Drupal.org accounts, not the git usernames.

The reason is simple: Everybody can provide a patch, even the users who don't have a git account. As far as I recall, everybody can have read access user to a Drupal.org git repository. You can create a patch that solves an issue; once the issue has been resolved, or the feature request implemented, the git commit will report the usernames of those who contributed with a patch.

The other reason for using the Drupal.org usernames is that not every users see somebody else's git username, and it is more practical to use what is visible in the issue where the patch is being discussed.