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They are probably made up of all sorts of pink/blue/green egg layers (Ameraucanas, Ameraucana mixes, ect), possibly different hatcheries doing it differently.
In my personal opinion they almost deserve their own breed standard. Like the Icelandic Chicken, they may be variable but you can almost always pick one out if you see a picture. That's what makes a breed, right? Being consistently different from other breeds.
Most hatcheries have stock that originated from the first imports of 'blue' layers from South America. Hatcheries have bred them for egg color and for increased laying rates, crossing in production breeds.
Purebred Ameraucana were refined into a recognized breed from those same birds. The big difference is that Ameraucana have been bred to meet a specific set of traits for many generations. Easter Eggers from hatcheries have been bred without any specific goals or traits in mind other than production rate and egg color.