At a guess, I'd say that it has more to do with header/manifold configuration. The header pipes are of different lengths to time the pulsing exhaust gases and I'll bet that it has a lot to do with the sound -- how those exhaust pulses are interleaved. Manifolds, obviously, are less well tuned and probably have the same effect, if different.

It probably also has a lot to do with the bore of the cylinders and the width of the exhaust path. I wonder if you fitted a wider but otherwise equivalent ehaust pipe if it would change the note?

But, like I say, it's just a guess.