A couple of comments on the pencil through discussions.
First, skin is elastic, so you rarely get an entrance or exit the size of the bullet mushroomed or not. It's a poor judge of what happened.
Exit wound size is determined by the pressure wave in front of the bullet, and bone fragmentation. Due to the different density constituents inside an animal the pressure wave may be large, small, growing or diminishing as the bullet travels through. Then add in the bullet shape and velocity are changing as well and every shot becomes a unique experiment.
As broz has mentioned, each bullet does its thing and we hope it performs under the conditions that we use it in. 100% is impossible in that set of circumstances.
We haven't even talked about bullets that impact and tumble. Which is actually what un-expanded bullets do inside animals. Once a bullet loses gyroscopic stability, it yaws and will always rotate, or tumble, around its center of gravity. There is a lifetime worth of reading on this from military testing of solid projectiles.
To sum it up, that's why we carry more than one round and reload for a follow up.
Jeremy