I'd like to find the citation I knw exists because I've got it bookmarked somewhere.
The way it developed is that Nokia developed what became Symbian S60 and S40. The S40 was a simpler version, without multitasking and other features, but was originally based on much of the same code S60 was based on. But they went their own ways, with S40 going into cheaper phones, and S60 going into "smartphones". It's like saying that Mac OS X and the iPhone OS X are the same thing, but different.
It this point, they've evolved so far apart that they've long been incompatible, but they started out with a basic codebase. There's been talk that the Symbian Foundation may re-integrate S40, but I don't know how far that's gotten, or if it will happen.
The way it started, as I remember, was that S60 was developed first, and shortly, S40 was broken off from it.