Presumably, in your first post you tried both devices on the same pair? If so, it is apparent that the VMG3925-B10B is showing slightly more loop loss than the HG612. But otherwise, the performance is the essentially the same, within a small margin.
I experienced the same thing with 8800NL vs 8800AXLR2 with loop loss (I believe the former was worse), though to a smaller degree. Point stands, variance between modems is normal. I have another 8800NL spare that I could try, and I have a feeling it is the better one (one that shows less loop loss). Not right now, but perhaps in the future.
Though, the QLN baffles me the most.