Just like anything, many variables can apply. Double lung seems to cause a quicker kill due to the sudden loss of oxygenated blood going to all parts of the animals body. Overall systemic failure of major organs due to such loss is the major reason for quicker kills over a heart shot. The heart being an extremely strong muscle can be pierced cleanly but because it is very fibrous muscle tissue can tend to seal itself on heartbeat expansion. Although this is the pressure cycle of the pumping heart it can also be the sealing cycle. Hitting the attached arteries will cause massive blood loss, more so that the pierced heart shot. This also tends to produce quicker kills. The brain controls all the body functions and again without oxygenated blood being supplied to it systemic organ failure and shutdown occurs. This does not take into account the variable of adrenaline induced flight. JMO & that of a buddy in the medical field that discussed this with me a few years ago. He is an emergency room, trauma surgeon who has treated many gun shot victums over the years. He was kind enough to share his knowledge and expertise. I might not have all of the details 100% but to the best of my memory my above statements are as accurate as memory of that discussion serves, presently. I hope this helps explain some of the variables with regards to a deer's death run.