I think if you look at all DAC reviews, a mismatch of .1 or .2 dB is quite normal. Even the original X16 shows a bit more than .1 dB difference.
As for your difference measured over all frequencies: you’ve just measured the passband ripple of the digital filter. If you select a different one, it’s gone. Well, not gone.. just smaller
That is not a defect, just a property of the specific filter.
Btw? How do you know your headphones are exactly matched? I bet they have a much greater imbalance than the DAC. Never mind speakers. The reason you can still pick out the difference: for one you seem to be good at it, and secondly, you’re probably very used to these headphones. Your mind wil compensate.