The "diff" appears to be backwards (it's subtracting what prettier would emit, and adding back the original).
The diff is between the test cases’s expected output (red) and the utility’s current output (green).
The red represents what would Prettier emits, and the green represents what Biom e emits. If you think that's unclear, feel free to send a PR to help us make it clearer.
This was intuitive to me, especially because it's in a section titled "Differences with Prettier".