Spine only comes from "the amount of arrow tube available to bend"
Effectively putting long pile inserts in reduces the amount the front few inches of the shaft can bend.
Tungsten is denser than stainless (
Mass, Weight, Density or Specific Gravity of Different Metals)
So for the same pile weight the insert will be shorter. Therefore allowing more of the tube to flex.
(The actual flexibility of the pile material is almost a non issue i would say)
Overall arrow length could differ somewhat due to pile design/shape/type
Even nock type/design could effect it, but it(overall length) will not effect the spine