Agreed, but I think alot of these chicks would hatch out just fine under momma hen. What I have found is if a chick has a longer (but still normal) zipping time than a styrofoam forced air incubator frequently shrink wraps them- no matter the humidity level. If I hatch that same chick in a cabinet incubator with laminar airflow- even at lower humidity levels the extended zipper will hatch just fine.
Sometimes it's just the artificial environment we put them in that creates the problem.
I have kind of come up with a theory that the humidity under a broody is a lot higher than that of artificial incubation. However, when she heard peeping and such, she looks under her to check on them, and probably adjusts the egg position so they don't drown like they do in artificial incubation. I have a broody cochin right now, she's not setting anything, but I think I will put a couple eggs under her and a hydrometer/temp combo... The kind with the external wire for outside readings and see what is really going on under there. Anyone ever tried anything like that?