I
think what Art means is that it's hard to judge whether yourself, as an individual, improved from one week to the next given the fact that courses are set up differently by different people and a difficulty baseline is hard to maintain.
As an example, I shoot with a guy who regularly shoots around 285/300. I shoot roughly 20-25 points less than he does from week to week. Two weeks ago, we shot a course that was much more difficult than we'd encountered since the beginning of the year. He shot in the mid 50's, and I stayed with him for the most part. It can be said that I had improved from the previous week even though my score wasn't as high as it had been in the past, especially since most everyone's score was down from their average.
Comparing your score to others' over a period of time is just a means of gauging self-improvement on a varied set of courses. At least this is what I think he meant!
Fritz