Hatchery easter eggers are not Ameraucanas that became mutts. It happened the other way round - the mutts became "pure" by selective breeding.
Years ago, people who wanted their easter eggers to breed true to color, took hatchery birds and developed color lines by selective breeding, then strengthened the colors by adding white skinned breeds like Orps to their lines, till they bred true. And called them Ameraucanas and started a breed club, at a time when the hatcheries called the easter eggers they sold "Auracanas." Then the hatcheries switched to this new name, Ameraucanas, and it has been confusion since.
The 'auracana' technically are rumpless birds and are in the lineage but add more confusion, being a third factor hard to explain.
Anyway, I've been reading BYC for years, at least back to 2003, and that is what I understood back then, but it is confusing, so I may have it all wrong.
The hatchery easter eggers are great birds, healthy, hardy, and lay dependebly. A mix of blue egg auracana originally crossed with brown egg layers with yellow skin making willow legs and large terrific turquoise eggs.
The Ameraucana club outlined their goals when developing their breeds colors of blue eggs and white skin and slate legs. Made great birds also and are working on more colors like lavender, mottled, and barred.
"Purebred" does not have the same meaning in bird breeding it has in horse breeding. In an Arab horse, for example, it is expected to carry actual genes from previous Arab horses. In birds, not a drop of historical genes need be present - it only has to look right, and the look can be recreated by mixing breeds. Look up Chanteclers and the 3 colors for an example.