Yes, I know. But try to explain it to the NSS crowd.
That said, stuff turns up on the SHO list for all kinds of reasons. Mechanical failure (yes, it does happen), human error (people selling and sending in certs that don't arrive at their broker's on time), transactions improperly recorded, requiring correction, and so on.
Another thing the NSS crowd refuses to understand is that fails to deliver can be long as well as short.