Why bullets work better than arrows on Exes

“The whole reason the bullet in the head works with exes is because the hydrostatic shock from a rifle round turns the brain to jelly. A pistol round bounces around inside the skull two or three times.”
“And turns the brain to jelly,” added Gibbs.
“Don’t some people survive getting shot in the head, though?”
Danielle nodded. “And so do exes, every now and then. Or they keep moving, anyway. But head shots are still the best bet, so that’s what I’m basing this around. And right now, this isn’t fast or powerful enough to take care of that.”

Excerpt From Ex-Isle, pg 63
Peter Clines
https://books.apple.com/us/book/ex-isle/id989619017