I breed and show Ameraucanas. I've observed thickness of muff and beard seems to correlate most strongly with particular family lines, but individuals can vary. Also, some true (per APA Standards) Ameraucana parents carry recessive clean-face genes so some of their offspring end up not meeting requirements to be qualified as true Ameraucanas.
Links that amazondoc provided are excellent for explaining differences.
Links that amazondoc provided are excellent for explaining differences.