Well, first it is NOT a sensitive primer going off from the bolt hitting it. The primer shows a clear firing pin mark.
And how very wrong is the assumption that a slamfire round won't have a nice deep firing pin mark.
I had two out of battery slamfires in Garands, the second blew the receiver heel into my face. Both cases had nice deep firing pin marks. No primer cupping either.
I had one inbattery slamfire with a AR with the new brass finish WSR. The primer mark was indistinguishable from any other round I fired that day, no primer cupping.
And the second shooter on my firing point, he had a AR slamfire, with Federal match, and his primer looked like any fired primer. Nice deep primer mark. no primer cupping.
In a recent Rifle or Handloader magazine, Mike Venturino shows the back end of a 7.62 Russian slamfire case. Not a shallow primer mark, though on his rifle, there is some cupping.
I don't know why this is so, but it is so. Maybe someone can install a high speed camera in the bolt face and figure out the mystery.