I have to leave the "why" to knowledgeable people and can only raise additional questions (perhaps comprised in what 3) is dealing with, IDK).
Above a few kHz I believe that we're leaving the "pressure chamber condition" range and that local features start to become increasingly important, so even the angle / position of the drivers / earcup / whatever start to matter. Perhaps this might (or might not, IDK) explain the pretty big 6kHz or so spike that I'm experiencing with all samples of the Hi-X65 I've measured so far, regardless of the mic type :
https://www.audiosciencereview.com/...hi-x65-open-back-announced.23380/#post-900059
I can only speculate here, but this among other reasons is why I'd raise the question of the anatomy
around the pinna, which is incorrect on all ear simulators so far unless you have Fernando Alonso's neck muscles (and will vary quite a lot between individuals anyway).
Regardless of the type of headphones (even larger, open ones), I tend to take everything above 5kHz or so with a big pinch of salt, regardless of the ear simulator or the operator. It's only when a common feature between all types of ear simulators and across various operators repeatedly occur (ex : Focal's 6kHz spike) that I reliably experience something of the same ilk to some degree (which is also why I greatly value some form of diversity in ear simulators measurements),
With ANC headphones the feedback mechanism, which locks the response below 800-1kHz no matter what - as long as a sufficient level of seal is obtained - but leaves it to vary above, and perhaps combined with some extraneous sensitivity in some regions of the spectrum because of some intentional choices related to the overall acoustic design, may make them more susceptible to variation in the ear canal gain region compared to other headphones :
https://www.audiosciencereview.com/...d-fi-and-sean-olive.27017/page-12#post-935561
One of the reasons I believe that it would be preferable to attempt to estimate where this variation is the most important and leave out a few EQ filters to let individuals vary this region to taste.
Rtings will soon publish their full QC45 review, which exhibit channel imbalance in the
exact same range where they are the most susceptible to compression, and which IMO can be entirely explained by the above phenomenon (and not necessarily by a gross manufacturing defect per se, as only a couple of mm of compression can make them wildly swing around 2-4kHz, indicating a design very sensitive in that region).
Note here that pad compression is only used as a proxy to estimate susceptibility to several variables at the same time, as it's impossible to control all but one of them to test each one of them independently of the other, so it's not a very good test to determine the cause of what's happening.
And then you add on top of that other idiosyncrasies related to the electronics, which can also bring inaccuracies, for example either because of volume-dependent EQ (Bose), if the ear simulator measurements were made at a different volume you usually listen at (quite likely in my case), or the behaviour of Apple's Adaptive EQ's algorithm when ANC is turned off (or in the case of the AirPods 3 when they don't even have ANC), which requires a broad signal such as noise or music to effectively work, and which may lead to errors when measured with sweeps. It's possible that other ANC headphones have other forms of idiosyncrasies, Rtings's review of the JBL Tour One ran into this problem and apparently couldn't find a solution in time for publication :
https://www.rtings.com/headphones/reviews/jbl/tour-one-wireless