Q:

Why does the audit system think this is a great answer? 

I just failed an audit of the "First Posts" queue, by downvoting this answer on "Alternative for “tail -f” that follows filename". 
This answer is not correct (as observed by one commenter), and does not add anything to the existing answers (including the answer I wrote, and which I remember).   Although it was posted before my answer, it was only a subsequent edit that added any explanation, otherwise I would have merely corrected it rather than posting a similar new answer. 
Why shouldn't I have downvoted? 
Additionally, the fail message has only the option "I understand".   In this case, I don't understand (hence posting here), but there seems to be no option for that. 

A:

Ramhound's answer is very thorough.  I'd like to clarify his answer to "Why shouldn't I have downvoted?" 
There is no reason to not downvote a technically incorrect answer.  In my opinion, therefore, you did the right thing.  Downvoting wrong information is good inside and outside review queues, even if it was posted before a similar, correct one. ( Though, if you recognize a question in a review queue, it might be a good idea to use the link link on the right to get the full picture.) 
Remember, the purpose of review audits is to stop "robo-reviewers", people who just click buttons randomly to get badges.  The audits are randomly selected, and sometimes the system picks bad examples.  One failed audit every once in a while won't hurt you. ( If it makes you feel any better, even moderators get audits, and fail them occasionally when they're dubiously chosen.)