I used white noise, not sweeps. It was my setup at the time. Since then, i’ve gotten a much improved interface and the hump is gone. I assume the hump was from stray capacitance, or the size of the cap on the input of the old sound card. (The test setup in the shop is too far from my recording setup to use the good stuff, so it was fairly janky by comparison until a couple years ago).
You can see from the plot of just the test setup in that folder that the hump only appeared with the DUT in the path, hence the resonance theory.
Also keep in mind that many of those are linear plots rather than log, and low end levels relative to high end are thus higher than a log plot. On the log plots, they look pretty close to what one would expect in most cases. But the hump was there, no doubt.