Neil,
it's well-known among audio pros that the external audio resolution sampling* on PCs is reduced by a factor of
ten, compared to industry standards, due to digital truncation [so that 44.1 KHz becomes 4.41 KHz and 128 KBps becomes 12.8 KBps]. This results from the original PC designers' failure to anticipate its use for musical storage and source reproduction and providing only eight-bit registers for audio.
This explains why Macs are still massively dominant in professional recording studios and the film industry, and why all speakers sold for use w/ PCs are so tiny [and tinny].
Just Kidding!
Actually, I have no idea how to explain your observations...
Kb
* this term is sometimes referenced by the acronym, "EARS"