Null testing/difference extraction is very sensitive to frequency response and delay differences. That is why audio recorded from actual analog DAC outputs, which do not have perfectly flat response, and also have small random delay variations due to the clock frequency not being entirely constant (basically, very low frequency jitter), will produce a relatively high difference signal. Notice that the sound card recordings all have a notch in the difference signal at the 1500-2000 Hz range, because that is what the frequency of the tones used for matching the levels and delay was. In the case of D.wav, the minimum phase filter has a group delay that is zero at DC but increases towards the Nyquist frequency, and therefore the difference is also greater in the high frequency range. This file also has the highest group delay in the treble range.