Q:

Why do review audits change the answerer?

I just failed this review https://stackoverflow.com/review/first-posts/14974720. But in contrast to what one sees now, this answer was presented as beeing posted by a new user. I already knew this question and that it has exactly this answer, so I tried to downvote and and mark the answer as a repost of an already existing answer by a new user (which in my opinion was the appropriate action to take, correct me if I'm wrong). But already the downvote triggered the audit to fail.
How should one know in a review queue whether a answer is an audit or a repost by a new user if the username is changed? I even rechecked with the question to see whether it is a exact copy but since it looked to me as if two different users were involved letting this answer pass feels wrong to me.

A:

The user is changed because if you saw a post from a user with 10k rep, you'd know it wasn't a post by a new user (which is what the queue is there for). This would be a dead giveaway that it was an audit.  Trying to find good audit posts from users with < 10 rep would be really hard, because audits are chosen from posts with lots of upvotes, and those upvotes would themselves give the user enough rep to make the post not qualify for the queue, even if someone's first post did happen to be that good.

How should one know in a review queue whether a answer is an audit or a repost by a new user if the username is changed?

You can simply go to the post itself and see if it's a duplicate.  You should be doing this any time you suspect a duplicate post, rather than flagging a post entirely off of your own memory anyway.  In this case, going to the post brings you to the top accepted answer, so you know it's not a duplicate of itself.

