Oh, I agree. I just thought it was interesting that I haven't had more losses yet, from my older eggs. Like you said - thousands of variables can impact it. Another thing to consider with the age of eggs, is for shipping. It seems that the older the egg (and thus the larger the air cell), the more likely it is to suffer internal damage in shipping and be unable to hatch (or even start growing). So it makes me wonder if maybe older eggs (up to a point) might be more affected by minor things that don't bother fresher eggs (such as temp and humidity varations, etc.), and that might account for some of the losses.
Just thinking out loud.