I am not sure but it probably uses your currently logged-in
GitHub accound.
Try the following:
Log out of discourse
Log out of Github
Log in to GitHub with the account you want to use on
discourse.
Log in to discourse using GitHub.
Then, hopefully, you will be logged in to discourse with the
account you want. Now you can go ahead and log back into GitHub
with the account you want to use for your GitHub work.
This is just one of those shortcomings of GitHubs shared login
feature.