ORIGINAL: Straightarrow
Believe me, I don't want to soud disrepectful, but there are simply a host of reasons this might happen, and not indicate which will penetrate furthest on an animal. First, arrow tips must be identical shape and size, arrow diameters must be the same. They also both have to have perfect dynamic spine while being shot out of the same bow, and they should have the same percentage FOC. Here's the big one - the medium shot into, has to be of different densities which will tend to deflect the arrow. This is what happens when an arrow hits the edge of a bone or at an angle. The forces of defection have to be resisted. Heavier arrows will always resist these forces best. High FOC arrows will deflect less than low FOC arrows. This is the primary reason why tests into plywood and steel drums might be a good indication of broadhead strength, but have no real bearing on penetration potential in an animal.
I may be thinking incorrectly but I don't see how this test couold be possible. Using the same arrow diameter and foc can be accomplished , but increasing the weight is not. That would entail jumping a cuople sizes up of the same arrow (i.e. Beman 400 to 300 to increase the weight enough to get 100 grs more) and keeping the diameter and foc the same by using the same fletching,tip,and nock, but the spine would be way off. Even if you could increase the weight of the shaft itself the spine changes.
You would have to get an arrow manufacturer to make 2 arrows w/ the same spine that were 100+ different in weight to run a test like this.
So I think we can conclude that spine and foc as well as mass and speed contribute to penetration. They are all functions in and of themselves. The variables(I don't believe) can't be eliminated.