+ 65 grains in broadhead weight would definitely alter the arrow's foc significantly. It would also require a stiffer arrow spine.... unless someone was already shooting an arrow too stiff for his setup.[&:]

The likely reason you see so much difference between a 2315 and 2413 is the different arrow diameters. 2315 is only 25 grains heavier (assuming 30" arrow) and 1/64" smaller and it's stiffer spined than the 2413, but adding all the differences together... It's not the tiny bit of extra weight itself that's causing you to see any difference in POI out to 30 yards.

I've experimented with adding weight inside carbon shafts (to keep arrow spine and diameters constant, eliminating variables) and, even adding 150 extra grains, they shot to the same POI at 30 yards as the unweighted ones did. Beyond 30 yards though, that's a whole 'nuther ballgame. But I tested at 30 yards because most people limit themselves to that distance for hunting shots, and most others should.