Exactly! You got it. A common misunderstanding is that the betas show the current state of the latest & greatest build. They don't. You only have to look at the differences between the GM and the final public release version to see how far ahead Apple's internal builds are. (That's why I treat GMs as betas in my chart all they represent is the final version of the APIs for app developers to test against. Apple is just saying, "We won't change the APIs any more after this one.")
Another reason, I suspect, is that there are unannounced new features in the OS, that have no public APIs for app developers to use, but they need to keep them out of the developer betas. They can maintain two parallel branches of the code for only so long, then they have to merge them together and test the result before public release.
GoldMaster, GM, is what they ship to manufacturing to be installed in the devices shipping and sold to customers. By definition, GM is final and production ready. Or thats the intent, no one is perfect.