most bolts use the top three or four threads to hold the load, strain, torque the fastener is trying to clamp. Studs are suppose to distribute the clamping load across all the threads, not the top two to four threads Another thing to consider is the studs are threaded on both ends, course bottom threads like stock and fine threads above the part being clamp or held, which is providing more clamping surface per inch of threads than the course threads do
When I have found a head bolt(or any other bolt) that starts to stretch to much before reaching the torque values needed the bolt has stretch on the top two or three threads getting thinner there which is telling me it pulling apart at those threads
IHTHs