It only considers a branch that you've contributed at least once. It compares the situation between the start of the branch and its head. It then reviews it for any new problems, i.e. those that have been added on to the branch.
Existing problems, i.e. those existing at the start of the branch, that have been solved, are shown as "fixed".
PullReview also examines files containing new changes made in the branch. The actions to fix them are shown in a second list Seize the opportunity to make the code base better should you want to do some clean-up work.
The integration branch (as defined on GitHub as the repository's default branch) is treated differently, as there is no starting point to compare with. In this case, we simply show all existing problems on that branch.