It’s when the left and right speakers are different volumes or have different characteristics. It’s extremely common with Hifiman headphones.
My first pair of Ananda Nano headphones had about a 8db difference between the left and right speaker. The left speaker also didn’t have the same amount of sub-bass as the right.
The 2nd pair I tried had the same issue but at a lesser level.
This is a good test to notice driver mismatching or if the headphones will pop/clip or rattle during bass.
https://youtu.be/J0a2Prc_MQo?si=oK6d2WvMQx2vc0DT
Listen to bass tests at a higher volume and notice if the hifiman pop/clip. Mine did and no other headphone has ever done that to me before.
The treble attack on hifiman is harsh and bright to cover the crappy mids and make them seem more detailed and analytical. To me they are just overall torture devices and unpleasant in pretty much all possible ways.
There is also a problem that planar headphones lack impact, so you want to turn up the volume to get the same impact as dynamic driver headphones at lower volumes but when you turn them up all you hear is the harsh treble because the mids are poor and overall recessed. If you EQ to tone down the treble the headphones will just sound even more lifeless and dull.