Nothing unfair about it, the truth is everyone got that question wrong (because there was no right option available), so they didn't fault the test takers, instead just annulled that one and scored everyone based on the actual questions, it is as fair as it can be.
It does suck for the people who though they had that one right, it is distressing to be shown one score and then later find out that the real score is lower. That should have been avoided by having an official recourse window before publishing scores, so when problematic questions are annulled then people only see their actual final score (in my country a recourse window is standard for all public exams).