The second part of what Yesitsconcrete said was that there are many variables that also affect cracking. I believe this is why sometimes one slab will crack and a seemingly identical slab will not crack. I would suggest that you look carefully at the differences between the contractors methodology between the slab that cracked and the one that didn't. Was there a temperature difference at the time of the pours? Was there a difference in the amount of water added? Pouring concrete in a very hot environment like you are in can tempt a contractor to use a lot of water in the mix. My limited experience as a do-it-yourselfer suggests that pours with more water are more inclined to crack.