Headphone measurements vary most between individuals at high frequencies.

Evidence

Møller et al. (1995a) measured the transfer characteristics of 14 headphones on 40 human subjects at the entrance of their blocked ears and found the frequency response was characterized by smooth structured fluctuations at low frequencies with low variation among subjects up to 3 kHz where it increased with frequency. At 8 kHz the intersubject standard deviations were 3–5 dB as the frequency of the resonances shifts among individual listeners. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1738