Is this not a fad or a trend, but the logical consequence of 5128 measurements?
In the course of the research leading up to the 5128, B&K found that 711 couplers deviate a bit from the average ear canal / eardrum / middle ear impedance they obtained :
Earlier studies also seemed to show some inconsistencies between 711 couplers and average impedance from a set of individuals.
The geometry of the "acoustically + physically average" canal they obtained also is different from the simplified geometry of 711 couplers. The geometry they obtained is a little bit different from the "physically average" geometry obtained from
that project, but mostly quite similar in the most important parameters (volume, length). Type 4.3 (5128) in green, Humanshape data in teal :
It's quite likely that in absolute terms IEMs frequency response has indeed been measured a little bit "wrong" compared to what really happens, on average, in actual ear canals. That applies to the entire spectrum, but in regards to this thread's concern in particular, for the 200-4kHz or so region it means that compared to a target derived from the same methodology, either with a 711 or 5128 fixture (ex DF HRTF + tilt and / or shelving filters), you can expect the error curve to be a bit different. Exactly how "right" type 4.3 / 5128 are I don't think we know - and the 5128 specifically seems to have some issues at lower frequencies (rocking modes + a sharp dip at around 400Hz that may not occur in actual human ears) -, but it would take quite a solid argumentation to claim that they're worse than 711 couplers in terms of acoustic impedance or physical geometry.
An IEM equalised to the aforementioned 5128 target would then, when measured in a 711 coupler, look like it has a bit less "gain" above 1kHz if you normalised at lower frequencies, but in reality it's quite likely that it's the lower mids that are over-estimated in 711 couplers.
Of the IEMs mentioned here only the Mega5est and Da Vinci seem to really follow that "meta" in the mids, the other ones would all still exhibit quite significant deviations.
We're only talking about average ear canal (and impedance) here. Your own ear canals may be a bit different from the average either in terms of volume, length, eardrum impedance, etc.
I would not necessarily call it "new" though - some companies were probably well aware of some limitations with using 711 couplers when measuring IEMs several years ago, and a handful of companies are now in a "post average ear canal" world as they're evaluating IEMs' behaviour in a set of ear canals, not just an average coupler, and designing active systems to compensate for individuals to individuals variation, or even individualise the response in the first place (Bose, Apple).
