Consider that the iOS 8 GM was issued in mid-August to coincide with volume production of the iPhone 6 and 6 Plus. 10 million iPhones preloaded with the OS for a mid-September release don't just magically appear overnight. It's not like Apple stood still after they issued the 8.0 GM and waited until the bug reports started coming before getting to work on the fixes.
As others have said, this update addresses the bugs that Apple already knew about when iOS 8.0 went gold. The next update will address the bugs that are discovered post-release.
FWIW, in my experience so far iOS 8.0 has been a lot more stable than iOS 7.0.