I agree with your statement. You'll notice that there are certain topics that are heavily concentrated on on the GMAT (e.g. number properties). Since they appear very often, it is a very very good idea to focus heavily on them because you will be able to nail them as they come. This means more correct answers and if you're lucky (or just extremely well prepared), more correct answers consecutively. I feel that I got very lucky on my exam in that I had a lot of number property questions and I had a very strong mastery of them. The ones that the GMAT threw at me were so easy to me that I actually that I was doing terribly (think 400-500 range).
If you have a weakness on a heavily tested topic, imagine how you would score:
Hit, hit, miss (weakness on heavily tested topic), hit, hit, miss (same topic), hit, hit, hit, miss (same topic).
Furthermore, I do not now whether the algorithm also tracks the difficulty bin by specific topics. If it does, then your score drops even further - imagine you miss a 600-650 num prop question then the algorithm gives you a 575-625 num prop and you miss that also then it gives you a 525-575 and you finally get it right. Again, I am not sure if that is how the algorithm tracks the scores, but do you want to take the chance?
