When Pac lands a punch at JMM, JMM's head is steady
JMM was moving away from Pac's punches - which were overreaching most of the time and not connecting at the correct proximity. That's why JMM done so well. He stripped away Pac's ability to land the way he likes to. He was forced to settle for punches that, at best, were landing at the tip of his reach. They were grazing shots that aren't easy to spot on TV unless you slow things down. That's probably why the compubox stats were in Pac's favour. The better, more noticeable shots were coming from JMM, whilst the grazing, unclean punches in higher numbers were Pac's.
Also, it's important to note JMM's head rarely ever snaps back from punches. He has a pretty stiff neck. Whilst Pac, with his floppy hair, can be spotted a mile away when he's hit. Shit, even the ones he blocks with his glove make it look like he's been smacked by a hayemaker.