Unexpected yes, did you recover the bullet? I don't think I've ever had a bullet not pass through on direct broadside shot through the ribcage. All the way up to 535yds. But on other occasions have seen some hunters bullets not pass through on close shots.

But if the bullet goes into one side of the animal and sticks in the hide on the opposite side, then that bullet has dispersed as much energy as it could and a pass through would have not gained anymore knock down power. But on the other side of the coin people have gone to higher velocity loads or magnum calibers to gain pass throughs so as you mentioned easier tracking. Bullets that tend to open up to fast will not have the penetration that the slower expanding bullets do. But also putting the slower exspanding bullets in a high velocity load will alot of times leave the animal standing in front of you just as if it wasn't even hit at all. This being the case on most broadside ribcage shots. Shoulder shots will tend to open up the bullet and delilver the initial knock down energy because of bone and muscle mass.

From what I have seen personally on animal hits and bullet penetration most bullets will have pass throughs under 100yds. Even through both shoulders on elk and black bear. Pistols and muzzleloaders need a harder bullet for deeper penetration because of slower velocities. High powered rifles do best in the medium range of exspansion. Not too fast or too slow. If a bullet will open up but will retain atleast 75% of it's weight under a 100yds on bone contact then it has substancial energy deposit with enough energy for reaching the opposite side and possible pass throughs.

Example a Remington Core Lokt is a softer bullet which will work well in a heavy bullet that moves slower. Take the same bullet and reduce the bullet weight and add velocity your going to have explosive bullets on bone.

Nosler partion is one of the best because of the partion inside of the bullet that will stop expasion and retain bullet mass. But the front of the bullet is not heavy jacketed so it will open up on contact and provide knockdown power.

The Failsafe bullet is reaching the other side of the spectrum in which it massive jacketed bullet is made for penetration. While it still opens up its just a couple steps down from a full metal jacket if loades to high velocity's. Slower exspansion leaving less initial knock down on impact.

But the final part broadside shot through the lungs and heart. All this talk on what bullet doesn't mean dinky doo. lol. Just one out of three animal mights travel a little futher to die. End result the same dead animal.