it's also possible that although merit varies widely in many fields, only a few have developed effective methods of performance evaluation.

I'd lean more towards this being the truth. In my experience, the people setting the salaries (HR) have no clue how to judge the effectiveness of the technical employees, and thus rely more on outside factors such as searching for salary ranges.

I have personally been lectured by 3 HR managers at 3 different jobs that I am "within the expected pay range for my job" and then pointed to Yahoo! Jobs or whatever is the fashionable website at the time. (Of course, this is generally the point at which I start looking for a new job.)

At each job I find myself less and less expecting of merit-based incentives. No matter how above-and-beyond I go, they don't materialize. I could have demonstrably saved the company hundreds of thousands or millions of dollars, but thanks to readily-available pay ranges it's all too easy for the bean-counters to throw up their hands dramatically and exclaim how there is nothing they can do to help me out.

RE: Salary Increase By Major | WSJ