The refs/pull/<number>/merge is a reference created by GitHub to keep track of what would happen if a pull request was merged. It references the merge commit between refs/pull/<number>/head and the destination branch (e.g. master).
You can think of it as a “future” commit. GitHub (as well as other Git-based collaboration platforms) use this technique to determine whether the pull request would successfully merge with all the checks passing and no merge conflicts.
Atlassian has a good explanation of the reasoning behind this server-side implementation detail here:
When you view a pull request, you’re seeing what the resultant merge commit will actually look like. We do this by actually creating a merge commit behind the scenes, and showing you the difference between it and the tip of the target branch.
They’re obviously talking about Bitbucket, but the same applies to GitHub.