Delta bed levelling

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I have an interesting one here I cannot seem to find online anywhere.

Apparently, the most needed adjustment to level a delta is offsetting the delta radius to compensate for a concave or convex bow in the z-height, from outer points at the towers to the center of the bed. Ok, so I'm all set with that; my z-height is very even at points "C" "X" "Y" and "Z". So given that, everything should be great, right? wrong.

at points "F" "L" and "R" I am evenly, consistantly, 2mm too high. :-/

how can this be? Something with the math is not adding up. How can I be level at center and outer poles, but be off at the outer extremes between poles?

What value can affect this?

Everything I can check and test tells me my values are set correctly, even after my manual calibration of this machine and z-height, my values perfectly match the seemecnc firmware posted just days ago.

So what am I missing here?

Thanks!
-Dave