People can and do fudge the numbers, but like CMF said the amount of numbers coming in help eliminate that. We are THE (total herd enrollment) in the Simmental breed, and what that means is we have to submit all birth, weaning and yearling weights if we still own the animal, and get to register the calf at a discounted rate. Last year ASA started a docility epd, and in just a year the number is pretty darn accurate. At weaning, producers access the calf temperament going through the chute and give it a score of 1,2 or 3. From that small piece of data, they have a docility score. I can tell you that sires that have a low docility number have a tendency to have loopy calves (nuts!), so it worked in that case. Your highest accuracies are on your AI sires, since there are producers all over the US submitting data on those bulls and have no reason to lie (they do not own the bull).
AFA carcass data, I think that bull owners send steers and heifers to the feedlot that then collects the carcass data and gives it back to the ASA, so the numbers are represented by those animals. Also, there is the new DNA data they are using that improves the accuracy of the data.
Hope that helps!