Not really when you think about it...
If you consider an unmodified twin needle carb, calibrated for glow but used for gasoline, both needles can be set accurately for idle mixture and full throttle mixture, but when opening the throttle coming off idle, the idle needle lifts off it's seat at a too rapid rate. In original design, that rate of lift is constant (the slanted groove is a straight line).
Now regardless of whether the carb is modified "airbleed-style" or "grindstone style", in both situations, at idle, the idleneedle is in the same position relative to the spraybar: the position needed to pass the required amount of fuel for idle RPM.

My mathematical theory is very rusty, and my algebraic skills are even barely existent, but what IS interesting, is that the top of the curve you found empirically, more or less coincides with the inflection point (where the ground part of the groove transitions into the straight original shape of the groove), basically eactly how the top of a mathematical function coincides with the inflection point of its integral.

I have noticed this correlation of "derivative" and "integral" quite often between process control values, and control element values.