What was the field scoring average today?
That would be a better measure of course difficulty, as opposed to one man's efforts.
I think Kim's score demonstrates little more than the fact that when today's pro players get hot, they can go low. Especially when they're as talented as Kim, and putting well.
Matthew:
The overall scoring average for the course probably is reflective of lots of things, including conditions (weather), pin placements, overall set-up (tee positioning and length, e.g.), to name a few major ones.
My theory/argument is that Kim's 11-birdie round, and Campbell's nine birdies in 15 holes, suggest the course has evolved to the point where singularly great rounds are now (again) possible, in a way that they didn't seem to be a few years ago. ANGC was known in the 1970s and 1980s (and really into much of the 1990s) for great low rounds -- often in the 4th round -- by players being aggressive and/or on the top of their games for that one round, i.e., Player's 64, Floyd's 65-66, Jack's 65, Price's 63, Tiger's 65-66 in 1997.
One of the reasons players struggle at the US Open, it seems, is not just because of the traditional USGA set-up. If they're lucky, they only get to see the Open course twice in their career. But the Masters is unique, in that players return year after year. Has it simply taken the players a few years to figure out ANGC changes, and thus their familiarity with how to attack it has yielded the likelihood of more low rounds?