Since you seem to be on knowledge quest I will try and confuse you even more
. There are two types of spine related to arrows. I may get the terminology wrong, but you will get the point.
One is static spine, which is what I described in my above post. It is simply a specification given to a particular arrow. Like lets use a 2213 for example since that is what I shoot. It has a spine specification of .460. So if I use a spine calculating program and it tells me I need an arrow of X amount of inches with X amount of spine I can just look at arrows with that spine. It is independant of length, it is just a way to classify arrows and their relative stiffness.
Then there is dynamic spine. This is how the arrow reacts to your particular set up. How it flexes when it leaves the bow and on it's way to the target. Which is why we need the arrow charts and spine software. There is no real measurement for this. Which is why we do all sorts of testing and tuning. If you take a given arrow, such as the 2213 depending on how it is set up the dynamic spine will be different. Things that can effect it are length of the arrow, tip weight, fletch and nock weight, cam design and draw length. Even little things like what type of release you use, if you nock under the arrow or behind it (string loop), nock travel and how well your bow is tuned can all effect dynamic spine to a slight degree.
On a spine tester the length of the arrow and tip weight doesn't matter (as long as it's long enough). But with a bow a longer arrow with a heavier tip will always have a weaker dynamic spine than a short one. Which is easier to bend, a 3 foot rod or a 1 foot rod?
This is why two people with the same bow set at the same specs may require a slightly different arrow, or different adjustments to get the perfect spine.
Paul