Some have theorized that the bolts which cracked were overhardened, and that those that weren't overhardened  will stretch, with the single locking lug acting as a lever, but not cracking, until the guide rib bears too.  That's someone else's theory, perhaps plausible.  I do know that lapping the locking lug to get the guide rib to bear can create excess headspace, depends on how far one has to go.  My Lt Col friend did that with a new bolt body, I have it here, and his rifle with my barrel has another new one he bought to correct the headspace problem he created.  Locking lugs must be softer than the receiver, he lapped it but most all material removed came from the locking lug.