I've noticed while watching cylinder head reviews of all kinds with flow charts, that there seems to be a somewhat fixed general range in the lower lifts no matter it's size, total cfm, valve size etc.. if it's a 300 cfm 289/302 sbf head or a 540 cfm big block Chevy head.
You are correct and there is a perfect explanation for it. I know you like numbers and math, so do this: Read up on Bernoulli's principles in articles such as this
It's dry, it's boring, but I promise a small light bulb will turn on when you (somewhat) understand it.
Here's the key. On the flow bench, the depression (the difference between pressure at the inlet to the port and the pressure in the cylinder itself) is usually set to 28 inches of water, or about 1 psi. So that gives you a certain amount of energy (or driving force) to work with. There is a limit to how much flow you can get. Bernoulli's equation helps define that limit. Dig into it.