I can only conclude that these off-flavors are as much in the heads of the judges as they are in the beers themselves.
Yes and no. As social animals, we are remarkably susceptible to the power of suggestion. If you have a pair of judges, and one blurts out 'Diacetyl!' while judging a beer, the other judge may suddenly pick it up (or convince themselves they now pick it up). It's how performance 'hypnotists' stay employed. Nobody is actually getting hypnotized, but the power of peer pressure makes them act like fools so they aren't the jerk ruining the fun.
That being said, the common off flavors/sensations are certainly real and trained judges can try to minimize those sorts of biases and tricks entering into their evaluations. And for me, my top 5 most-encountered beer problems are:
1) Phenols (cholorophenols being most common due to water chemistry in the area)
2) Bitterness, as in harshly overdone
I encounter diacetyl, acetaldehyde, and DMS pretty rarely. I've met plenty of people that claim to be super sensitive to one or all of them that get it practically in every beer. I try to avoid judging with those people.