Threads / psi (I think this is the correct term) is the difference. For example, lets say that we can screw a stud in to max depth (where it bottoms out) which is 20 threads. Until there is an object such as a cylinder head to be torqued down, there will be 20 threads engaged at 0 psi. Once torqued to spec, all 20 threads will share the load.

 To screw a bolt in, even the appropriate length one, You will start to exert tension on it once the head of the bolt makes initial contact with the top of the object to be torqued. This means that at initial contact, you may only have 15-16 threads engaged. At 50% torque load, you may only have 17-18 threads engaged. That leaves 2-3 threads still unused while the remainder of the torque is being applied. In theory, this is why the threads become stripped more easily when trying to torque a bolt as opposed to a nut on a stud.