I’m not a butcher … but I did work in a meat packing plant in Detroit years ago. Though I worked mainly in the cooler and freezer, moving around quarters and sides of beef etc and filling orders, I did also spend time in the cutting and grinding rooms. Yup, all kinds of trimmings went into the auger/grinder for the ground beef along with a predetermined amount of fat.
However, I’m going to guess that it is not fat content that made those great burgers.
The difference may have been what the steer ate … or more accurately, *where* the steer ate; that gave the meat it’s flavor.
So, you might try to find *not* just "organic" hamburger, but actually beef that is said to be "free range" … that is, the steers were out in the field eating grass for the vast majority of their diets, not pellets in a trough.
Other than that, maybe the breed makes some difference … try for a good Angus beef (?), but I’m going to bet that what made the difference was actually what the steer ate.