For example Didn't Jack let go the mastiff that he fought a few chapters ago?
Why would he let go a mastiff he had a strngle hold on but jump on a disoriented and fatally wounded guy's neck as hard as he could?
even the bite to the carotid artery is out of character because just the previous bite was already enough to damage shingo's battle potential.
underground tournament jack would have just finished him with punches instead of trying to go for a gratuitous "coup de gras" by biting the carotid of a guy that he could just as easily punch.
Jack is about efficiency. It's not like he bites because he just loves biting, he bites because he thinks it's an under-utilised way of fighting . He used it to useful effect with the first bite to shingo's shoulder, but the second "finishing bite" to the carotid could have just as easily been a finishing blow with a haymaker.
Not my Jack.