There are two main issues here:
First, it's very difficult to accurately measure the frequency response of headphones. You can use a "HATS" (Head And Torso Simulator) but every human has different shaped ears, and ear canals. And when the entire "environment" a headphone is operating in (especially an IEM or earbud) becomes just the ear, it's a lot like putting different speakers in very different rooms--it will significantly change their sound. So if you use the same HATS for all your measurements, and if you always position the headphones exactly the same way, and if you keep everything else the same, you can reasonably compare the measurements to each other. You still can't compare them to what your particular ears and brain will hear, or even to "flat".
Second, subjective preferences run wide. Some like lots of bass, some like bright exaggerated high frequencies and many like both--that's why lots of headphones have what many call a "V" shaped response curve--it rises at both ends. It makes the sound more exciting for many people. Others like more accurate sound--look at the response of Etymotic headphones as one example. Someone who buys say Monster Turbines is likely to hate Etymotics and visa versa. It's all about subject preferences.