If I completely remember that part (and I may not) Graham and Merrill assumed the person (alien, whatever) outside was on the ground. Then, when they ran around the house, he was on the roof. For him to do that quickly, they assumed he had to have jumped. I think they might have been wrong; the alien might have been on the roof before they even ran outside. (and in that case, there could have been lots of ways for it to get up there without needing super strength)