Sennheiser HD 600 spec sheet's diffuse field frequency response looks a bit strange?

[HELIAS]

The following five pairs of headphones have official frequency response curves from Sennheiser, compensated to their diffuse field reference:

HD 600: https://assets.sennheiser.com/downloads/30ff65e5871381f39f8c6d11c90e8f03.pdf

HD 800, HD 800 S: https://assets.sennheiser.com/global-downloads/file/6582/HD800_HD800S_Manual_08_2016_EN.pdf

HE 1, HE 90: https://assets.sennheiser.com/global-downloads/file/7510/HE1_Manual_0917_EN.pdf

The HD 600 frequency response stands out to me. The other headphones' graphs generally agree with third-party measurements, but I can't draw similarities between the HD 600's curves given by the manufacturer and reviewers. I expect the curve to be much smoother, roughly following a -1 dB/Oct tilted line, which is not what I see from the spec sheet.

Any reason behind this?

For reference:

Crinacle's Headphone Graph Tool | Hangout.Audio

Compare industry-standard GRAS 43AG-7 headphone graphs, measured by Crinacle.

graph.hangout.audio

Squiglink - IEM frequency response database by Sai

Compare hundreds of frequency response graphs between IEMs and earphones from manufacturers like Moondrop, Sony, 64 Audio, Fiio, and more.

sai.squig.link

 

[ZolaIII]

And manufacturers reference should be for which exact model (year)? Old initial high impedance driver whose a bit better as I remember. Fix on finding such with familiar HATS and ear model from third party to the grain of salt. Then try to compensate.

 

[HELIAS]

And manufacturers reference should be for which exact model (year)? Old initial high impedance driver whose a bit better as I remember. Fix on finding such with familiar HATS and ear model from third party to the grain of salt. Then try to compensate.

The spec sheet was written in 2004. Quite old.

 

[AnalogSteph]

The spec sheet was written in 2004. Quite old.

In fact, I'm fairly certain that the measurement itself dates from the '90s and might be as old as HD600s themselves...