"The Block" may not be the best material to shoot into.

If some of your shots are going more directly right between the layers of foam instead of hitting the edges as they enter- that can alter your findings. A fresh, solid foam block would be better for each shot- or better yet, ballistics gel. And just doing three shots to make up an average may not be enough either. How about 10?

If the spine of the arrow is reacting poorly to the heavier tip, that will degrade your penetration findings also.