Quoting Barnes:
You are going to have to explain this one, because it doesn't seem to make any sense.
Sure. Two things.
First. The way the beveled shoulder on the stud, locks against the beveled opposite shoulder in the head forms a very tight bond. So tight you could consider it a semi weld. It's not really a weld mind you, but the purpose of that bevel is not just to bottom the stud shoulder out, but to lock it, effectively making it one with the head. This does a few things. One, it makes it a very strong structural component of the head itself and two, preloads the threads of the stud. This bevel and the "semi-weld" characteristic, is of vital importance to the stud itself and partly why it has superior holding strength to a bolt.
Two. Why is the stud of equal size and material stronger? Easy to answer. Threads, therefore surface area. The beveled shoulder has already locked and preloaded the threads on the stud. So all effective force is now between those two surface areas, that is, the entire surface area of the threads and the constant force trying to push them apart. The same holding force would be applied if a bolt were used of the same length however it would not have the benefit of the shoulder therefore it would only load the threads to the torque of the bolt. On the same bolt, on the head side, there is only one surface area. The bottom of the head of the bolt. That is all the holding force that bolt will ever have under any circumstances, ever. The bottom of the head of the bolt. On the stud however, You have both, the bottom of the nut AND all the threads of the nut. More surface area=more holding power. Simple math.
That is a paraphrased version of the explanation I received from Corky Bell when I asked the exact same question.
/brox