Ameraucana alliance website has a lot of information and explanation on this topic both in the forum and achieved newsletters and books. That is one of the great things that drew me to Ameraucana the original developers of the breed are still around and very active in preservation of the standard. A lot of the not mixing varieties has to do with the genetics used to develop said varieties and once you start crossing them you quickly move away from the standard and once that happens it’s hard to get the genetic to align again especially if you don’t know what you’re doing with genetics.