> neutral sound means that all frequencies are perceived at the same volume. i.e the sound doesn't lean toward one nor in the other direction.

That's exactly it though - you get a rotary button to control the amount of bass, and you get asked to dial it in until it sounds correct, as in "it sounds the way it should sound".
You repeat this with many, many people go get a meaningful average.

"neutral" does not mean "flat on a measurement".