I have a theory about this but it is very much in the realm of creative thinking and speculation, possibly even fantasy, definitely not science and please keep in mind that I have a kooky sense of humor. Here it is. I think maybe when the dark brown coating is being applied to the egg, PERHAPS...
I responded earlier but now have something to add. One of my Marans laid eggs like OP's for several days in a row. To describr one, it looked like a landscape painting, with areas that were much darker than others in erratic patterns, reminiscent of mountain ranges. Sorry I took no pictures. It...