Interesting :
Most modern ANC headphones (and that includes the Airpods Max and Momentum 4) have a feedback mechanism that ensures that the same response is delivered regardless of coupling issues in the range where it operates (below 500-1000Hz depending on the model).
In the case of the Airpods Max it seems to result in a pretty steady response in absolute terms when moving from 711 ear simulators to 5128 ear simulators, at least up to 800Hz (to be noted is that it's expected to see a difference above several hundred hertz / 1kHz because of the different fixture).
In the case of Soundguys' M4 results, while the shape of the 50-400Hz region is slopped similarly to 711 results, it's significantly misaligned with the 400-1000Hz results.
Difficult to know more about what could have caused it without additional measurements to characterise where the feedback range operates (up to which frequency).
The recorded channel imbalance is also quite interesting :
There's a pretty noticeable step at... 400HZ or so. The high-Q difference around 2-3kHz is the sort of channel imbalance that I've seen so far most frequently on some ANC headphones which also have a very non-linear behaviour under pad compression, ex here (QC45, H910N) :
https://audiosciencereview.com/foru...se-use-of-the-harman-curve.29633/post-1087051
TLDR : it's the sort of situations where I'd rather listen to the headphones for myself to pass judgment on their bass response. Looking at the 711 measurements, in comparison to the Airpods Max, the M4 looks like they'd have far too much bass, while looking at Soundguys' results they look shaped quite similarly to the Airpods Max in the 100-1000Hz range.