That would be the simplest explanation, but I think a complicating factor would be that for the Nazi concentration camps, many people were murdered, healthy and unhealthy alike, alongside those who perished by other causes.

One might also hypothesize that survivors tend to be more careful, conscious of their health, etc. I don't know if it's a great hypothesis, but it seems plausible.

not really. That's a conjecture that may or may not be true, but it certainly isn't simpler than small sample statistics tending to be extreme.