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Probably to make it resonate better.
Joseph Zisa did initial testing and volume calculation (I don't know if he actually did any calculations or if he was just performing trial-and-error fest, I can't find anything about this design but thingiverse item).
I'm not an expert in acoustics, but it was clear for me that air intake is somewhat fucked up.
So I improved this flaw and we now have this version.

I tried altering chambers a bit, but anything other than this ratio sounds less loud.
Be it research or trial-and-error, sounds like Joseph did this part well.
Sadly, we don't know why and how.