I'll still argue the difference was at record time, and in the sample rate conversion done. The final sample rate and bit depth isn't going to be the significant factor. The way frequencies are recreated is based on sine waves already, so even tho the digital representation of it is smoother, the final output will be the same. Where it's possible you'll get differences is from harmonic distortion introduced by frequencies greater than 22khz but it would take a hell of a signal path to recreate those correctly anyway.
I'd guess that the variability you hear is in the DAC design, and the optimization for specific sample rates. Chances are it's better at some frequencies than others, and that's the improvement you're hearing.