Sure sounds like stretching going on there.
Let's see some pictures of the failed bolts and a link to what you purchased. You might have a few hardness tests done.
Not to second guess you, do you know the torque wrench to be calibrated and you were reading it correctly?
I had a neighbor ask to use my compressor to fill a bicycle tire recently. He read the wrong scale and was trying to put 5 kg into a tire that holds 50 psi. Point is, mistakes can happen.
