I don't think you're entirely correct either. I'm extremely glad that you've had such amazing luck, but it doesn't mean everyone is going to. Also not all chickens behave the same way and where you live greatly impacts the probability of them getting worms/mites. I have personally experienced...
So what you're saying is, you don't need special food so long as you go out an buy calcium and oyster shells? also if you're telling someone to feed the shells back to the chicken you should at least inform them that the eggs need to be finely crushed otherwise you can end up with chickens...
I would say that's very normal. It might take them a few weeks before it gets to be routine. Also make sure you have them on layer feed otherwise the eggs will continue to be smaller