Maybe i am picking this up wrong, apologies if i am but... you are assuming the "unloaded" wheel is lifting up the same amount of the "loaded" wheel, it doesn't, no matter how much we say ignore the flex, we can't it is always there and to be honest it's not a constant either in my eyes on our very basic roll bars. (nearly but not quite).
If we did have perfectly stiff bars with zero deflection then for me it would make no difference on the length of the arms of the ARB as long as they were mounted at equal points on the wishbones, but as they do flex i think that the further away the wishbone pick up point is on the bar to the central, chassis mounting points then the softer and "less effective/more flex" you will get, i think this is only natural and it is something i play about with from time to time, obviously it is a tiny adjustment but i believe it is noticable.
I hope this helps or maybe just adds fuel to the fire
