[Formulas]

Flow is constant on a flow bench, Flow in a real engine it is pulsing ( stopping starting ) with each valve hit

You have pockets of air / fuel pushing and pulling each other ( pressure differentials ) making their way through the intake tract ( simplified )

Seemingly useless open areas may be a good place to bunch up ( storage of higher pressure) waiting for a place to go on the next hit

And where the pulse is of lower pressure you would want minimal CSA to generate speed quickly

Pulses can be thought of like a frequency on a oscilloscope display that change with rpm