Nakakatakyut! Nag-upsample ka to 384kHz?
Empirical Audio puts it very nicely:
"When hardware is designed to upsample audio streams on the fly, there have to be compromises in order to fit the logic in the chip and do the upsampling in real-time. There is also the issue of talent of the designer in selecting the best algorithms, filtering and dithering.
Software upsamplers on the other hand have no real time constraints or hardware fit constraints, so the designers can use more complicated algorithms that cannot currently run real-time. These can be more accurate and get closer to the ideal hi-res master."
This and the fact that most interfaces exhibit lower jitter at higher sample rates may be the reason for the improvement.