The measurements are of multiple driver arrangements and the benefit of lower IMD from dividing the response by adding drivers. Even here, I would think THD would be lower due to more specialised drivers requiring less excursion for the same SPL at a given frequency. Regardless, almost all headphones (not IEMs) only have a single driver.Neumann (and S&R) shows drastic differences in IMD not tracked by THD, though; these example speakers would be the KH120A and KH310A, I guess. But I doubt we need more knowledge than "more ways better"; some one-time comparisons like 2-way vs 3-way coaxials or sealed vs ported would have been nice, though.