Exactly. That's completely different.They did not test for audibility thresholds for harmonic distortion, they tested for audibility of an added signal at various frequency, when playing music.
High degree harmonics on low frequencies will possibly be reaching the highest hearing sensitivity frequencies.
At the same time, high level of low degree harmonic distortion on low frequencies will probably be first heard as an increase in loudness, and there are studies showing that a tiny difference in level may do a big difference on subjective impression.
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