Ski tests allow you to get a feel for a ski's general characteristics relative to other skis. Unfortunately, we are almost always "comparing apples to oranges". Here are some of the factors I think about when I read a test review.
The tuning factor - I believe that the tune on the ski given to a tester makes the most difference of all variables in how it is perceived.
The category factor - I believe that the magazines establish testing categories, then solicit the ski companies to submit whatever ski(s) they wish to be tested in a category. This leads to skis of entirely different sorts in the same categories, partly by variance in the perception of what is requested of them, and partly by the ski companies' trying to "one-up" the others (by placing a ski in a category in which it really does not belong to out-rate the anticipated entry from another company (in forgiveness, for instance).
The length factor - probably the second-most critical variable to tuning, and you will see that the companies play the same games with what length they submit as what category they are in.
The conditions factor - a ski can be better or worse than its neighbor in given snow and weather conditions, and vice-versa in other conditions. Depends when the testing is done.
The tester factor - a ski pro may not perceive the same characteristics in the same way as you would (forgiveness = good or bad?, for instance). I believe testers exacerbate this problem by trying to review with their perceived audience in mind, and may rate a ski as "too unforgiving" for the masses even though it is ideal for very good skiers, etc..
The company response factor - some companies do not submit entries to one test or another, for whatever reason.
The judging factor - does the test determine numerical order 1-2-3 winners, does it use "gold medals" and not list or mention the "losers", or does it list all skis tested with a review?
The "Let's not irritate any of our advertisers" factor.
Even with these issues cropping up, I do feel that I can get a good comparison ski-to-ski by reading all the reviews I can get my hands on.