Hi Dabral,
Yes, thanks for the insight. I agree with you that some of the GMAC's question orders are not quite to be believed, although in general the questions in the OFG do increase in difficulty as they go. We are both tutors, and we see the same students getting the same questions wrong, so we know that something is askew.
Of course, the GMAT Prep quizzes will allow us to try questions of a certain difficulty level, as will the online question bank, but either way we're putting our faith in the GMAC to correctly classify the questions as easy, medium, or hard, and of course that is a rather vague system of classification in the first place.
I also agree that the GMAT Club members are smarter than the average GMAT test-taker, which skews the numbers quite a bit. At the same time, this information is quite useful to know, as a check against the accuracy of the GMAC's question-ordering system, and possibly a truer measure of difficulty, even if it understates the overall difficulty level of each question.
Another factor is that GMAT students who seek GMC explanations to easy questions probably get lower scores, and GMAT students who seek explanations to difficult questions probably get higher scores. If I am seeking an explanation to a question that 80% of students answer correctly, then I am probably a low scorer. If I am seeking an explanation to the hardest question on the test, then I may be an 800-level scorer.