I've found frequency response graphs to not be unreliable with headphones, just in my limited experience. I think they're much more reliable when it comes to Speakers and macro dynamics.

But everyone has different ear lobes (the shape of which affects sound waves), and different ears, and we all know how changing out the ear pads can make significant differences. So it is not only possible but likely that one pair of headphones would sound completely differently to two different people.