My chooks sometimes toww out a spotted egg, its like the pigment layer right before the eggs is laid just goes on a little thicker than usual. No biggie.
Both my Production Reds and Barred Rocks lay those on occasion. I have found that when I rinse them off, if I scratch at them, they come right off. I'm thinking mine come from the tract inside, just spots of stuff that stick to the egg on it's way out. It'd be cool, though to have large polka dots on them... yep... bet these city slickers would pay good for polka dotted eggs.....
If the jets get just a little clogged in the spray paint booth, this is what you get. Quality Control send them on through anyhow, because the inside meets spec, apparently.
All my girls will throw spots periodically. White spots seem to come primarily from the Wyandottes; the Wellies, while they normally lay uniform dark eggs will sometimes throw a heavily speckled one, or, even more fun, a "pinto" egg. The Java, before I ate her, laid a darkly-speckled egg.
I love them that way; brings even more color to the egg carton.