Stihl has been playing that game for many years. They come out with a popular saw, then they stick a bizarre chain size on it so as to keep all their customers visiting the Stihl dealership rather than the local hardware store. After a while, Oregon and the other chain manufacturers begin making the same chain, and then the cycle repeats.
I'm fairly certain that Stihl came out with .325 chain first, and after a while they upped the guage. Nobody else had .63 gauge links for a long time. While the .43 gauge chain might cut a pinch faster, it also wears out a lot faster. So... More money headed to $tihl.