The difference between some of corals could be growth. The captive raised OC and Rainbow Acans look different to me than a fresh wild piece. The wild pieces have that perfect round corallite, while the ones that start as frags, grow or even encrust onto the rocks and look different. I think that's why many people think that the OC and the Rainbow Acan are different from a regular A.echinata. But then, I could be wrong. My OC frags produce babies like a lord does, by encrusting on the rock, and springing up tiny polyps. I also have seen my piece have a corallite divide in half.
Here are my two pieces.
This one came directly from Tigahboy, but not Rainbow.
This piece came through a few reefers, but it's origin is from Mike at The Captive Reef.