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Torquing the bolts in the wrong pattern can warp the head. If it WAS flat, and then you tightened up the first bolt all the way instead of going around tightening them in steps, it'll no longer be flat.
Overtorquing the bolts can stretch them and the set will have to be replaced. You know you've over torqued a bolt and have to throw it away when it suddenly becomes EASIER to turn while tightening. The torque difference been proper temporary stretch and permanent stretch isn't that much. (Think of a balloon that has been over inflated - it will never again shrink back to it's original size.
The flatness tolerance is tiny because a head that is flat sitting cold on the bench can not be flat when torqued and hot. Since any torque will distort the head in done way, it's critical to minimize that situation - torque in three steps, in the right pattern, to the right stretch, with bolts that haven't been over stretched.
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