Apple are a bit old-school about releases, they seem to bundle all major updates together and only give security and minor updates to their previous releases, and they follow that for all of the software they provide.

This is how it has historically been for 90's and early 00's unix-like operating systems like *BSD and Solaris. A lot of things come bundled with the base so everything in the operating system's base release is updated with a new release of the kernel that comes bundled with it.