Once you create a pull request, if you look near the bottom of the issue it creates, there is a message which mentions that if you push more commits to the branch you've made the request on, they'll be appended to the pull request.
All additional commits pushed to the same branch become part of that pull request. The reason for that is due to the way a pull request works. You can think of it as the person accepting the pull request as pulling the current state of that branch at the moment they decide to merge, rather than creating a set of changes and asking for that to be accepted.