Oh genetics DEFINITELY play a factor. Muscle insertion, fast twitch muscle fiber strength (or speed, depending on who you're talking to), shape and integrity of hands, etc. But fighters hate hearing that because they feel it undermines their work ethic.
Out of all my fighters, Bleu hits the hardest without trying to, and particularly with the right hand. This Canadian Cruiserweight I'm working with is the biggest, holding mitts for him reminds me of Arni, where I constantly have to consider he's one inaccurate punch away from breaking my wrist or fingers. But, the girl I train who is 110lbs, sounds the most impressive on the mitts because (as girls will do) she adheres to the form the best. Hence us calling her "Dinamita." I've been told by a guy who fought 3 people I train (and sparred 4) that everyone I train hits hard, but I can tell the differences between who hits WELL, and who is a gifted puncher.