I agree with a lot of what prodizy and FutureWorks have to say. And let me add..
The GMAT has to evolve over time, because it needs to be able to differentiate between examinees at the upper end. Its stated role is as a test of reasoning, which is problem solving, creative application of quant rules, etc. And in order to do that it has to continue to find ways to present the same concepts (algebra, number properties, etc.) in ways that look wholly unfamiliar to you. If you've seen that problem/problem-type before, you're able to solve it based on memory, but the GMAT can't just test you on memory...it has to test your ability to reason through a unique-looking problem.
So it's not really that the questions are "harder", it's that the questions look harder because at those upper levels of difficulty they have to look harder so that they force you to think, not just remember.
As an example - the question I'm referencing might actually be a live question so I can't replicate it (I heard it from an instructor who got it from a student and the instructor couldn't remember if it was his real test or a GMATPrep test). But there's a really tough question that one of our instructors brought up to two of us other instructors, and the beauty of it was that there's no known method to solve it...it looked impossible (but fun) on the surface, and the key (that each of us instructors found after 20-30 seconds of looking at it) is that you had to use the "complementary concept" (for example, division is a complement of multiplication; roots are a complement of exponents; etc.) to reframe the question to even have a shot. And once you did, you could narrow it down to two and have a 90% preference for one answer choice over the other, or there's an alternative almost-nobody-would-think-of-it method by which you could guarantee the correct answer.
I bring this up because once you've seen the answer I doubt you'd call it a question that's any more difficult than the hardest stuff on GMATPrep or the OG, but it looked like nothing we had ever seen at first. And that's what the GMAT is about - it has to force you to think. So if you're claiming that the actual test feels more difficult than the OG, in a way you're right, but mainly because the OG is something you've seen before, so you've had time to digest what those questions look and feel like. So by the time you've seen them, you understand what the "trick" is, and they're no longer hard...but if you were zapped by that memory-eraser from Men In Black, they'd be just as hard if you had to see them again for the first time.
****Weird analogy time - official GMAT questions are a lot like Mike Tyson's Punch Out on Nintendo if anyone played that. It's not that Mike Tyson was that much "harder" to beat than any of the other elite fighters...it's just that you saw him so infrequently that it was harder to catch on to the trick to beating him (dodge for the first 2 minutes, then you can counterpunch after that). The second Bald Bull and Piston Honda were probably just as hard, but you had more opportunities to see them and figure out their tricks, and their flurries where you couldn't touch them were much shorter. (Note: If you had the code - 007, 373, 5963 - yep, still have that memorized from the late 80s - you could fight Tyson anytime you wanted and you'd figure out how to beat him. Unfortunately, there's no such code for the GMAT that will let you see live questions.)***
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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