I understand that my arrow has an upward travel to about 18 yards at which time it starts its decent. What I don't fully understand is that although the two arrows are 72 grains apart in weight, they both hit about a foot below where I aim at 40yds.

Here's my best guess at what's happened/happening:

When I was shooting the 311gr arrow, I sighted it dead on at 25yds. When I switched to the 382gr arrow, I had to move my elevation adjustment down because the heavier arrow was dropping more causing a low poi as compared to the 311gr arrow. That's obvious I know. What's not obvious is why at 40 yards, the 382gr arrow only drops about a foot lower than where I aim. (about the same as with the 311gr arrow) If I sighted both arrows to hit dead on at 25, doesn't that mean that by the time the arrows reach the 40yd mark, that the heavier arrow should've dropped considerably more? According to the link, 10 inches more. But, that is not the case at all.

There's clearly a science here that just takes the right person to explain.