I agree that the way the actual GMAT makes questions "hard" is different from the way that test-prep companies, including MGMAT and frankly everyone else, make questions "hard." Whereas test-prep companies take known questions/topics and design more complex and confusing questions around them, the GMAT feels free to make up totally weird content for each new batch of questions, especially Number Properties questions.

For example, most of us here have seen the questions that rely upon knowing that n! + k (where k<n) will be a multiple of k (e.g., 7! + 4 is a multiple of 4). This is a pretty interesting and provable fact, but 5 years ago there weren't any released questions that mentioned it, and so you'd never see it mentioned in any test-prep course. Now, of course, I wouldn't even expect to see it on the test anymore, but rather some new random weird thing that I hadn't studied before.

From the test-prep companies' perspective, the reason for this is that we would be treading on dangerous ground if we tried to write our own weird questions on content the GMAT may never test. We have to stick to what's been officially released so that we can promise you that we are, in fact, preparing you for the GMAT, not some bizarre abstract math test. As a result, we create harder questions by adding to them layers of complexity and misdirection, but we'll never invent our own nonsense -- and you should be happy for that because it means everything you see from a legit test-prep company is a topic you KNOW will really be on the test.

What does this mean to a test-taker? It means you can't treat practice material as a source for concepts to memorize. The skill that the GMAT tests is your ability to see a weird problem and quickly deduce some way of dealing with it. When practicing, even if you DO recognize the Number Property being tested, pretend you don't: practice testing cases, plugging in example numbers, and figuring out an answer without necessarily knowing the theory behind it. The more you practice this skill, that is, figuring out an answer to a question that seems totally random and weird, the better prepared you'll be for the hardest questions on Test Day -- questions that none of us, myself included, will have seen in advance.