Because for the headphones I've had the chance to listen and to see measurements that have inconsistencies, the brightness due to frequency peaks shows in different ways than the one due to response peaks. Response peaks don't have a delay, some sort of echo effect which can be heard playing stuff at higher volumes.
I'm not saying their brightness is for resonance problem or delayed energy issues, just that to my ears, until measurements prove me wrong, I think the brightness is coming more likely for that than a pure response peak from the driver.