Eggshells explained?

[Bama-chicks]

We get this assortment often, just wondering what causes the two in the middle? I think the left middle is from the olive egger and the right middle is from one of two golden cuckoo marans.
Thanks!

 

[azygous]

Do you want the ultra scientific explanation of how eggs get their pigmentation? https://www.nature.com/articles/s41598-020-68070-7

Or the lay person's version of why some eggs are unevenly pigmented? https://www.backyardchickens.com/articles/common-egg-quality-problems.65923/