That DXOMark data right around ISO 200 almost looks like a measurement error? Photons to Photos shows no sign of the dip around ISO 200 in the Z7's DR. In the Photons to Photos data, the Z7 and D850 are right on top of each other at every ISO (as they should be - it's pretty much the same sensor).
Usually, DXO just looks like a shifted version of Photons to Photos - they have different noise thresholds, so DXO consistently comes up with something like a stop and a half more DR on every camera (the difference varies a bit by resolution, perhaps because of DXO's downsizing), but the curve shapes are the same.
In the case of the Z7/D850, they're within a couple tenths of a stop of each other (within measurement error) at every ISO in the Photons to Photos data, but the Z7 has a 2/3 stop dip right around ISO 200 in DXO's data...
I doubt that DXO would publish something of this nature if it were an error, because it is such an obvious deviation from the usual trend in DR graphs. The DXO team are experienced scientists. They would surely notice the anomaly and recheck the data before publishing.
The more likely explanation, as you've mentioned, is the difference in the noise thresholds that are considered to be relevant for the average photographer. 'Photon to Photos' apparently exclude the DR data below a certain threshold which they consider to be photographically irrelevant (ie. not nice), so they are applying a
subjective element.
DXOMark give you the full 'Engineering Dynamic Range', which I prefer because sometimes I want to see the detail in the noisy shadows.
The knowledge that the deep shadows at ISO 400 (with the Z7) will be just as detailed as the same deep shadows in an ISO 200 shot using half the shutter speed, could be useful in certain circumstances.
Of course, the ISO 200 shot will have better SNR at 18%, so that is always a consideration. However, when one makes a decision to use ISO 400 instead of ISO 200 it is presumably because one considers the shutter speed at ISO 200 to be too slow for an optimally sharp image, and one is prepared to sacrifice potentially lower noise for the sake of a potentially sharper image. The knowledge that deep shadow detail will not be affected, with the Z7, could result in one using ISO 400 more often, resulting in a greater number of sharper images.
It would be interesting if someone who owns a Z7 could do a comparison at ISO 200 and 400, shooting a scene with an exceptionally high DR, then raising the shadows in ACR and Photoshop.
