It's just lower noise.These are some very interesting measuring results you have posted here. Thank you for your work.
The IEM jack of the ADI-2 DAC seems to be the most distortion free headphone output currently available for power levels < 5.0 mW. Even better then all of the THX AAA models in terms of THD+N vs power. (Am I right?)
Currently I'm using it with 150 Ohm impedance headphones and it sounds very transparent. But the volume level of the ADI-2 has to go up to -20.0 to -15.0 dB for adequate listening levels.
And it's basically the same as the new thx 887 from monoprice at 0.6uV, being better than 1.0uV of 789.