Okay...so the bullet did a u-turn?
I don't know what the bullet did, but certainly if you're lying on the ground, trying to shield yourself from a guy pointing a gun at you, and the gunman is by your feet, a bullet could enter your lower abdomen from the side and exit through your sternum. Of course, it also could have ricocheted off a rib or two, but I don't even think that had to happen.
The description of the wound doesn't say the bullet went straight in the front of the abdomen and straight out the front of the sternum. There are all kinds of angles possible.