The embedded fragments were multiple.

They were not a single intact object.

They were the shattered remains of whatever had entered the rib — a tip that had broken into pieces on impact, with those pieces remaining distributed through the penetration channel while the rest of the weapon — whatever attached to the tip and gave it its delivery force — had pulled free or fallen away.

This fragmentation was, in retrospect, entirely predictable.

When a pointed object strikes bone with sufficient force to penetrate it, the tip of the object — which takes the full impact and must push through rather than deflect — is subject to extreme stress concentration.