That is actually an interesting point. Speakers play in a room, and the room factors into the total experience. In a sense headphones bring their own room. IEMs are maybe an anechoic chamber, and the acoustic resonance properties of the cups of the over ears can almost be thought of as a dedicated listening room. Which raises the question of how large the room should be, as well as what the reverberation attributes of the room should be. IDK, that anyone has done any research on this, or, if they have it's entirely in house to the companies and proprietary. But it would be an interesting area to explore.
For one thing, some people suggest Dan Clark phones sound like monitors playing in a studio, and often don't particularly care for the effect, maintaining they find the them dull or unexciting. I wonder if it has anything to do with the resonant (or lack thereof) properties of their cups? I know people might say that lack of such resonance makes them more accurate, but speakers are in resonant rooms as well, and those resonances are not on the recording either. Yet would most people want to listen in a dead room? Of course not, and there seems to be a widespread consensus that rooms need to have proper treatment, and there exists a large body of knowledge on how to design acoustically correct rooms with precisely desired levels and timing of reverberation. So in contemporary music listening, the rooms needs to bring something to the party to complete the experience.
So, is there a corresponding level of reverberation for headphone spaces that is preferable in the same way there is a preferred level for listening rooms? IDK, that anything has been quantified, and if it has, how could it be measured in an individual phone to determine how well the phone is performing? A measure along these lines (how sound behaves in a earcup) would, I think, be the headphone analog of the CEA 2034, and research in this area might go a long way toward explaining why certain phones with otherwise identical FRs seem to sound so different from one another.