Well done post, Shawn.
I was reading Wayner's problem. There's another explanation besides cross-contamination...
The temp charts you posted are annotated for simplicity. They're originally from a study that considered an 80% kill rate as "controlled", then "scientifically" (read, guessed at) adjusted for maximum efficacy. That's because beetles, just like most other bugs, become resistant to cold as they march north. Natural selection isolates the fittest, and they become more resistant. There's always a mutant bizarro or two in a batch of bugs that you just can't kill with freezing.
Wayner could just be THAT lucky.
Despite that, freezing lowers the odds of infestation dramatically. It's good practice, and as you've suggested, even better practice to do everything yourself.