I dunno, I think she just has a style that kind of backfires against certain opponents. She throws punches that don't pack that muck power – not because her striking mechanics are bad, but because she throws from so far away and thereby hits people at the end of the punch which makes for less of an impact. It allows her to keep the distance, which makes sense for someone who has great footwork and long limbs and does not want to be taken down.
When the opponent charges in, like Rousey did, Holm's punches pack a lot more power. But in most of her fights, Holly's punches just work as a set up for her kicks. She has some vicious kick KOs from her early fights, but up until the Rousey fight we had not really seen anything similar from her when fighting higher level competiotion (which is why the Rousey KO was kind of surprising to many people, me included – just goes to show how bad Rousey's gameplan was I guess). I haven't seen her boxing fights so I don't know what her approach was back then, but in an MMA context she strikes me as a fighter who is not that good when forced to lead and push forward. Which, incidentally, is what happened in her last fight.