Hi Joe, You have confused me yet again.
If it is nothing to do with spine or flex rate, why don't all spine arrows shoot the same?
When you go through a bow set up process what you are doing is adjusting the bow so that all (spine) arrows
do shoot the same. By this I mean that the flex rate of the arrow matches the bow geometry. The arrow goes through 0.75 of a cycle on the string and 0.25 of a cycle between the nock leaving the string and passing the riser. So overall the arrow goes through one flex cycle between full draw and riser and this takes 1.25 the arrow free free vibration time. Established/verified for last 60 years and forms the basis of all these arrow selection systems.
All spine arrows shoot the same. You select an arrow in the right ball park and adjust the draw weight to obtain this arrow /bow agreement. There is some wiggle room around this both with the nock/string separation and the arrow status as the nock passes the riser.
However....when you have a perfect bow set up, there is still a problem, the bow string generates a net torque on the arrow during the power stroke. This torque would result in the arrow spinning around a vertical axis as it comes off the string.(Try shooting a left hand bow right handed and see what happens, There is nothing to counter the arrow rotation from the string torque). The pressure button spring generates a torque on the arrow in the opposite sense to the string so it can be adjusted to get an approximate null net torque on the arrow. The higher the bow draw weight the higher the string torque on the arrow.
This gives rise to the well know relationship between draw force and button spring tension. (IIRC spring tension often quoted as 6 grams/lb draw weight).
If a flying arrow is rotating around a vertical axis it flies in a curve (vaguely similar to hook/slice on a golf ball). If the string torque is dominant then for a RH archer the arrow curves right (weak in the jargon). Reverse if the button torque is dominant.
For a given arrow rotation (energy) the deviation of the bare shaft arrow is larger than for the fletched arrow. So in principle if the fletched and bare shaft arrows hit at the same horizontal point then the string and button torques on the arrow balance.
Why does any of the above matter - well you can put together a reasonable argument that the forgiveness of the system (the arrow grouping) is related to the amount of rotation the arrow has when leaving the bow, and hence the string/button torque balance as well as the nocking point position.
PS There are no textbooks on archery, only books containing peoples' opinions (usually with zero supporting arguments) and these can be sensible or nonsense. The only real judge is how archers vote with their feet.
pps
Is this not all assuming that the arrows are the correct spine? If the spine is incorrect then surely by definition, the flex rate will be INcorrect?
If you cannot sensibly match the arrow flex to bow geometry then you have the wrong arrow. As for saying the spine is incorrect you could just as well say that the draw weight is incorrect or the archers arms are too long