The subjects tested on the GMATPrep are not exactly aligned with those of the 3rd party tests. So you may be better at the topics on those tests - even if other people insist that those 3rd party tests are harder. So for you the GMATPrep might be harder.

So let's think about the differences between the two:

The GMAT is edited and tested more than any other test. This allows better incorrect answer choices and more "tricks" and "traps" to be employed. 3rd party tests often involve more "straight math" and fewer questions that look simple but are actually very difficult because they lure people into these very careful chosen wrong answers.

I would say that you need to look to your techniques and figure out why you are missing these types of questions - those that appear simpler than they are.

I just responded to another question that is a bit similar and I think you could benefit from the articles that I listed there. Here is the link to that discussion. The articles are at the end of my posting. You might want to read my entire post there. https://www.beatthegmat.com/advice-neede ... 65778.html