It is well-known in the medical field that a direct hit to the prefrontal cortex will usually cause brain hemorrhaging leading to death, occasionally cause instant death, and occasionally cause no effect at all - not even permanent brain damage. Many areas of the brain are not vital to sustaining life, and not suffused with blood enough to present a risk of death from bleeding out. For this reason, a single shot to the head will have much greater chances of being lethal if the surface area of the traveling bullet is larger. Again, a tumbling bullet has a much larger surface area to make contact with than a bullet traveling straight through the brain. A straight bullet may only impact a few regions of the brain, and coincidentally do virtually nothing, while a tumbling bullet might create a hydrostatic shockwave that traumatizes an entire hemisphere of the brain, causing instant death.