Greg,
That is useful when the user is viewing ANOTHER person's beer list, and it represent that score. the "auth_rating" is still the score of the person you are looking at.
For example when you do /user/beers?access_token=GREGSTOKEN = you will get your own and auth_rating and user_auth_rating_score will be the same. However we recently added a new option to sort by your "YOUR RATING", so is now outside the beer object, called "user_auth_rating_score". This will change based on if you are viewing someone else's score.
It replaces the "auth_rating" inside the "beer" object.
Greg