MY money would be on it being an aftermarket bolt not made out of high/medium tensile steel ?
compare any markings on the heads of the good bolts to this one and see if they are different, a pic of the head of the stretched one and a good one side by side would be interesting
but you were lucky it didnt bust off inside the thread ...the way it has necked down it was sure close to doing that
by the way the necking is typical of freemachining / mild steel that has passed its yield point in tension
another quick (not definitive ) test is hold the bolts lightly by the head and hit the bolt lightly with another piece of bar ....an high tensile will give a pinging type sound and mild will give a deader almost thud sound ....if the sounds are different you can be 90% sure they are different material