This is what I suspect happened:
1) In manufacturing, parts are made individually and put into a parts bin for assembly.
2) there's a possibility there was a bad batch of bolts
3) this bad batch of bolts was inserted into the "bolt parts bin" and mixed in with the good bolts
4) as rifles are assembled, bolts are taken from the bin
5) over time the number of bad bolts diminishes but the possibility remains that a gun will have a bad bolt until the parts bin is completely renewed/emptied
6) CSA, not knowing which are the bad bolts and which are the good will handle bad guns by merely giving you a new bolt
Anecdotally, it seems that if you have a bad bolt the problem will occur in 300 rounds or less. So if you shoot 300 or more, your gun is safe.
Question for those who have gotten new parts from North Sylva - did they request you to send in your rifle, or did you just ask for a new bolt? How do they prove that you have a bad bolt? Thanks!