genetic do play a major role. In my own case, I can move a lot of weight for sets, ie, I've squatted 550lbs for 5 reps, ass to the floor, deadlifted 550lbs for 5 reps, benched 335 for 6, etc, but my single maxes suck. With how much I can deadlift for reps, it would seem like I would have a crazy max, but I've put up 600 once, and 625 once, thats not much more than what I rep with, but my frame just isnt built for big singles