From my shooting it seems to be inconsistent hand placement or changing grip pressure. Inconsistent light will also do it as your eye sees the sight differently as the light changes.
First thing to check is COAL !!! if not all the same, this will do that every time. Different lengths causes the crimp to NOT be the same if even if it is in a crimp groove or cannelure. Best to seat, in one operation, and crimp in another.
