I would have to agree Pete based on my very next sentence. I guess it should be It is pushed as hard as it is pushed?
One issue with the way your charting that is its anecdotal at best.
The C count is created by various means in a chemical paste. So the voltage variance changes from batch to batch and brand to brand. There would be little useful or reliably linear pattern to work with. I can still by a cheap crap high C battery and get a high voltage drop under load. My C rating my still deliver a certain amps for a moment or some may base the rating strictly on the IR but once the paste is fired up it doesn't deliver. That's why there is so much variance even within a brand let alone differing ones. Thats why you can have true equal C counts but the fall off can be very different.
And Ah, 0.3-0.5V difference between C counts how do you jump from 90amps100 buffered peak even with 65C batteries to a 10amp increase pushed to the motor? The mAh consumption under such are indiscernible when it comes to mAh capacity consumption.... A pilot could add an extra hard bank or one less and not notice a difference.