The shape of the ear canal varies from person to person. The length and diameter of the ear canal changes the acoustics of the sound environment. For the same reason a bell rings at a certain tone based on its size and shape, the ear canal can increase or resonate certain tones (Resonant Frequency) more than others. This is why some people hear a tone as light and shimmery while others hear it as tinny and harsh.
Insertion loss, or the amount of natural gain lost by inserting a device into the ear, has to be recreated with an in-ear earphone. How your earphone does this affects the performance. The resonant peak (the ear’s natural ability to amplify a certain frequency over all others) of one person’s ear canal may peak at 2k and another person at 3.5k. When you remove this natural peak by inserting something into the ear canal the sound changes differently depending on the person. The bottom line is that one device will not work perfectly for everyone. Enter the Variable Equalization (VQ) System.