I’ve found, and others I have talked with have found, that the POI of a fletched arrow varies unpredictably with spine. One would think that if an arrow is impacting left, weakening the spine should move the POI to the right, because that is what happens with a bare shaft. With a bare shaft, weak spine causes nock left causes POI to the right. The whole shaft moves to the left of the bow at first, but since the bare shaft is pointed toward the right of the target, that’s where it ends up. However, the feathers introduce another variable into the equation, because they correct the flight of the arrow. So as the fletched arrow moves downrange, the feathers correct the flight path. Depending on how effective the feathers are at correcting the flight path, a weak fletched arrow that starts out nock left (and with the whole arrow deflected to the left of the bow) might end up impacting left, center, or right of the target. And changing to an even weaker shaft might result in a POI to the left, center, or right of the first arrow.