Probably has something to do with the air speed or how the air is actually moving in the pipe. My bet is with air speed though. I'm honestly not sure
why it works that way, I just know it
does work that way.
BTW: A well designed system would minimize heat gain(using plastic, reflective material, heat wrap, etc.), of the air in the pipe. The air is also not in there long enough to make a huge difference, at least not in street use. Now if you plan on racing professionally....