If conflicting changes have been merged in, GitLab/Hub will tell you that your branch has conflicts and prompt you to update/rebase on the target branch. That doesn't happen simply because your branch is out-of-date with the target.
There are two types of conflicts: literally changing the same lines of code (which you're talking about) and conflicts of business logic between features/modules (which I'm talking about). What if your PR depends on a database table that was dropped in the most recent commit? If your branch is based on an old commit, your tests might pass, because that table has not been deleted yet from your POV. But there will be a failure when you merge.
