Egg shell quality - guineas?

I got some new eggs to set - guinea, french black copper maran, and brahma. My question is what causes the coating (calcium carbonate?) to spread unevenly on the guinea eggs. The seller assures me they are fine and to set them anyway, so I will trust their experience. I am curious though, as to what might cause it? A couple of the eggs look like they are missing spots of the outermost layer (it's cream & rough underneath?), almost in a mottled pattern across the fat end. Some eggs look like tan paint was dripped on them (no, it's not poo). I'll try to get pictures later if that might help. Could it just be an older hen?

Thanks!!