My analysis was gleamed from here...
http://www.zerofast.com/torque.htmThe requirement for the 75% stress factor is fatigue life.
Story goes, years ago, wheels used to pop off and roll off down the street by themselves cause they were tired of black cars and wanted to roll with color. Well the owners of the black cars got pissed off and complained that tires were breaking the studs in 2 cause it couldn't be the color of the car. The car mfg's looked at it and found that the tire would rest all it's weight on one stud to break them one at a time (someone must have told them the story of how to break a bundle of sticks). So how do you keep the wheel from putting all its load on one stud? You tightened them all down real tight so that the wheel has no room to move.
Ok, really... there's 2 types of stress, static and dynamic. Static, there all the time and dynamic, moving, fluctuating, changing all the time. Static failure in a bolt would be like a head twisting off from torquing it down. Dynamic failure would be from snapping off while driving down the road.
This might give you a little insight as to why our head cover bolts twist off upon retorque. The life of a bolt is acummilative. You torque it down, it goes for 20 years bouncing up and down. You twist it off without much effort. Torquing it the 1st time takes a little life. Bouncing up and down takes more. Taking it off and putting it on just isn't in the picture.
(excuse me I've got to go replace some bolts) 
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