See, I don't know everything, and I'm not a speaker/headphone/driver designer, but I wonder... might headphones and even drivers be tuned differently even if the drivers look identical?
I haven't read much about the HD518 or HD515, but it was commonly said that an HD555 could be made to sound the same or almost the same as the HD595 if a taped-on felt ring was removed from the HD555. Now, with the HD558 and 98, some people say a simple mod produces a similar sound, some people say it makes them closer, or that you don't end up with the same result at all.
What if the plastic membrane used for the driver looked the same, visually, but it was shaped differently or the magnets were arranged differently, or it stretched differently, resulting in a different frequency curve. These are potential unknowns. Something that does make a difference, a higher the price usually relates to more stringent driver selection. This may mean the two earcups will perform more symmetrically, or that a driver is higher performance and closer to a target performance. That could also explain some sound differences.
In the end, the PC37X totally surprised me because it didn't sound at all like I imagined from HD598 reviews. If anyone could figure out how to tweak a headphone driver in different ways, it's Sennheiser (and experienced modders).