I've wondered about that at times too. Sometimes my hens' eggs have really hard shells and particularly thick inner membranes. On the other hand, there are times when our flock has had a few members that lay eggs without shells (membranes intact) or with very weak shells. I have always fed the same basic rations and nothing about their environment changes except the seasons! The only thing that changes is the birds. We tend to re-home the hens that have spent one winter with us and raise new replacement chicks for them each Spring. A few select "pet" hens stay with us each year. But generally, 95% of our birds are new each Spring.
I have found far fewer egg problems with the standard heritage-type breeds...RIR, barred rocks, sussex, delawares. The hybrids are different. I find more odd eggs...misshapen, no shells, tiny, ginormous, etc... And we have had a tiny bit more reproductive problems with the hybrids such as internal laying (usually from a hen that has started to lay eggs with no shells) and prolapse (usually those that have layed ginormous eggs).
Be happy for those "hard to crack" eggs (especially when they're in your pocket!)