@CL14715 again made a very important and correct comment here. I want to put here the link about the article which I read not so long ago here on head-fi about measurements due to the conversation about reviewers reliability and value of measurements. I believe it will be very beneficial to read for anyone who is interested in understanding some technical aspects behind the sound.
https://www.head-fi.org/threads/headphone-measurements-the-new-standard-part-1.937301/#post-15735920
So, one can argue that this review is related to speakers, but actually type of the system does not matter, and the main things stay the same:
1. Even the most advanced systems of previous generation, that cost thousands of dollars, are far from perfection (aside from the average ear vs your own);
2. Although IEMs are inserted into your ears, which greatly decreases wave interference and some other issues of speaker systems, due to the complexity of human ear structure, simple microphone cannot show the frequency response even close to the real one;
3. Even smallest differences and nuances can lead to significant changes in the frequency response;
4. Again, although some improvements have been made, and this new system is much more advanced than previous ones, not to say just some microphone setup, this new system is still far from ideal. And this is putting aside all psycho-acoustic, biological and other factors that also affect the final sound you hear.
5. If you try to argue that yes, your system is not perfect, but it can be used as relative comparison of an IEM list - you are wrong again. Who told you that shift from the "ideal method" (although from modern science perspective it does not exist due to the specific nature of sound perception) is constant for every IEM? Small spoiler - it is not
Scientist myself, I always laugh when someone is supporting reviews with measurements, stating more "scientific approach", searching for the "ideal frequency response" or relying on such measurements for the purchase. Such people are extremely delusional. Don't get me wrong - if you want to spend your time doing this - you are welcome. The world is just a huge sandpit. But for those who put on a serious face, stating that you are ding some serious stuff and trying to convince others that you are right and they are wrong - sorry to dissapoint you, buth this is something like building nuclear reactor with lego. So, please, do not refer to such opinions as analytical or scientific in comparison to just "what I hear" ones, because they are not. Creating a data array with some statistics is not equal to scientific approach, as it lacks background of fundamental research behind and is with 99.(9)% not reliable and misleading. Unfortunately, most of the people who are not in the academic sector have very approximate understanding about what the real science is.