I grew up on a production egg farm back in the 60's and 70's (not at all like the modern, horrible chicken houses, our girls were happy judging from the way they sang). We had about 10,000 White Leghorn's at any given time. We would get maybe a couple of dozen soft shells a day even though the laying mash had oyster shells in it. I think it is just a thing that happens sometimes. BTW if you get to it before it has a chance to get dirty, nothing wrong with using it, just have to do it quick cause they dry out fast. I used to scramble them and feed them to my "pet" roosters for a treat. We were supposed to kill the occasional rooster that got included in the flock but, my Dad liked to hear them crow and of course, they looked like Foghorn Leghorn so I liked them as well. We ended up segregating them in a cage of their own at the head of each aisle and they became my pets.