This is an interesting question. I regret that I don't know the answer for sure, but I can say that sound does not travels in any substantially different way upwards vs downwards. Rather, I believe the answer has to do with the fact that low frequencies are carried through the physical structure differently because of where they are: things that make sound tend to be in physical contact with the floor, not the ceiling, and that physical contact carries extra low-frequencies.
Imagine a stereo: it is physically connected to the floor (perhaps through a bookshelf) which conducts the low frequencies downstairs, and creates a muddy sound. Upstairs, however, they don't get those additional low frequencies, just the air->ceiling->air sound, so it's a bit clearer (albeit quieter).
The same logic would apply to children running, furniture moving, and even people speaking.