All sorts of possibilities. The two which come immediately to mind:
- that engine happened to be particularly well balanced, so values near zero didn't adversely affect it as badly.
- another safety check in the software that said you had to have at least one engine powered while in the air. That is, the fourth engine can only fail for actual mechanical reasons, not just sensors and if it does fail, you try to start one you took offline for sensor if any such engines exist.