@chungjun , long story short and highly over-simplified:
For ages, music was recorded to be reproduced on speakers. Somewhere along the line, headphones came into existence. Headphone manufacturers wanted to tune their headphones to sound like, how a set of neutral-stereo-speakers would sound, because music were after-all recorded with speakers in mind. Headphone manufacturers started developing their own headphone-target-curve, that was derived based on their philosophy & definition of neutrality. Due to variations in headphone-target-curves between manufacturers, a reference headphone from one company, would not sound the same as a reference headphone from another company. Apart from the lack of a universally accepted standard for a headphone's neutrality, there is also the difficulty of designing a headphone that can accurately reproduce the intended frequency response. Just another one of those things that is easy on paper, than in practice.
Now what Sonarworks did was, they arrived at a headphone-target-curve, based on, what they think is how a set of neutral studio monitors actually sound. Then they measured the FR of many popular headphones and derived EQ correction curves for each headphone, which then applied, would match the Frequency Response of SonarWork's headphone-target-curve.
So, True-Fi should not be seen as a tool that aims to make all headphones sound the same in terms of FR. But, rather a tool to correct the stock frequency response of a headphone, that is widely considered not-correct. But that doesn't mean that SonarWork's frequency response is the right one. Its just their version of what they think is correct. And its upto to the users' preference as to, whether they like the tuning of True-Fi or not. So its better to test the water using the trial version, before purchasing it.
Hope that helps!