No one ever said we were all using the same year, did they? Even so, the year doesn't mean the planet was created 600 years ago. In Ashbury, that year was the year of the establishment of a certain human culture. It's an arbitrary number. The Ashbury elves don't use that number when talking about their society which is thousands of years old, for instance.
Seth, there is a middle ground from your "everyone must follow specific orders and have no variation" and "everyone can do whatever they want". The middle ground is that races agree on their origin -- which is something that was decided long ago by previous Owners and National Plot people -- but culture is left completely to the chapters.
If your biata aren't from stone elves and gryphons, then you've created a new race, and that's not allowed. Biata and stone elves, after all, are not traditional fantasy races like elves and dwarves and ogres -- they're unique to our game. As such, we need to define them more than we do for the races people already know. And if we're not all defining them the same, then that's a big problem. It's important enough that we even put the origin it in the Rule Book. Wring a biata origin story where they are not descended from Stone elves and gryphons goes against what is in the Rule Book.
Come on, there is plenty of room in the National race packets for you to have great fun with creating cultures of each race without you making up brand new races that technically don't exist in our game.