I'm just wondering... do you personally find truly difficult problems to be moving targets? i.e, truly difficult problem A pales in comparison to truly difficult B, therefore A is truly difficult no more.
No, I try to avoid that trap. I've seen many a grad student avoid submitting their work, since despite years of battling with a problem, once they understand it, they see it as easy, and therefore not worthy of publication. So to be clear, I may no longer find problem A (or similar) difficult to solve, but I still consider it to be a difficult problem, because I am cognizant of the path taken to solve it. Problem B may now be more difficult for me, but I may not consider it to be the more difficult problem, since the path taken to solve it appears to be simpler. Essentially, in this context, it makes sense to consider difficulty subjectively, both from the perspective of those have solved difficult issues in the field, and also from the perspective of those who haven't.
Like master, "truly difficult" doesn't have a standard definition. Personally, I consider difficult problems to be those that would take a smart person with baseline education (say, a bachelors degree) and baseline experience (say 5 years), years of high quality effort to solve. The "solving" process here would typically be research and familiarization with the right academic tools, as well as gaining enough of the right experience to finally achieve the necessary skills and insight to produce bleeding edge solutions. Of course, future work in this area would be easier now that the person has the relevant background - whether one considers someone with this education and experience as a master or someone in the 95th percentile of this group to be a master is arbitrary to me, since the term is nebulous at best, however, I do believe that the term implies a rare level of skill and the ability to now do something that the majority find difficult (and will for the foreseeable future).
This idea that "everyone in this world has an example of how difficult their job can be", so they are basically the same, is an utterly inane concept.