That is a touchy topic with some people.  I'm not aware of any hatchery that actually sells true Ameraucanas.  To be an Ameraucana, the chicken has to meet specific standards in recognized color or pattern.  It also needs to meet other specific standards, including eye color, leg color, body configuration, comb type, muffs, and a lot of other things.  Also, if a hen, in needs to lay blue eggs.  Green or brown is not allowed.  These requirements are found in the "Standards of Perfection" for each and every breed.  On this forum, the chickens that do not meet these standards of perfection for Ameraucanas are called Easter Eggers or EE's for short.
The hatchery chickens are sometimes called Ameraucana, but they just don't follow these standards.  They do not try to keep the different recognized colors and patterns separate like they do with the Rock, Wyandotte, or the other breeds.  Their pullets often lay green or even some shade of brown egg.  The hatcheries are selling chickens that should look pretty and lay colored eggs.  They are not breeding for show.  
The Ameraucana Breeders Association put has this comparison of what makes and Ameraucana, Araucana, and Easter Egger.  It highlights the differences.  Strangely, to me anyway, they call Easter Eggers "Americanas".  Notice the difference in spelling.  Talk about needlessly adding to the confusion!  
http://apa-abayouthpoultryclub.org/Edu_Material/Easter Eggers vs.pdf
To further add to the confusion, many hatchery EE flocks are based on Ameraucanas, often initially crossing Ameraucanas with other breeds to improve egg laying.  If you check the history, Ameraucanas and Araucanas were developed from blue egg laying chickens originally from Chile.  Just like every other breed that has been developed, some people wrote down some "standards of perfection" so they could show their chickens and compete against each other and then developed chickens to meet those standards.  Because the hatcheries used Ameraucanas to start their colored egg laying flocks and many still call these Ameraucanas, many people believe an EE should have the qualities, such as leg color, as the Ameraucana.  They can have those characteristics, but they don't have to.  
Some hatcheries are better than others with many of the other breeds, but their other breeds don't generally meet the Standards of Perfection for that breed very well either.  Their breeding methods are set up to provide mass production of chicks that somewhat follow the standards, not breed chicks for show.  Even the best of breeders that carefully pair specific roosters with specific hens to try to get that "perfect" show chicken have many more rejects than perfect chickens.  But at least most at least try to keep Partridge Rocks separate from White Rocks or Barred Rocks.  With their EE's, the hatcheries don't even try that.