I have been wondering why folks don't like to answer my posts as well...
So I will at least respond.... I don't know what causes the different pigmentations of the eggs, but I get some pretty interesting variations. I've had some with bumps and nodes, some that change color from one side of the egg to the other, and some of my brown eggs actually have the coloring rub off if I have to really scrub them to get clean. Its weird, but I think pretty normal. I've even got some with indentations and 3D rings!
Interesting read. I give a layer feed and free choice calcium and have not had thin shells, so I don't think such is the case here.
As far as the pattern on the egg, I am thinking more along the lines that the hen got interrupted while laying the egg just as the speckling function was started. She had to stop laying and retain the egg a bit longer therefor the speckling is so concentrated at the end.