They are two different code trains.
As bugs are fixed, they are usually backwards patched in to earlier major releases (1.3, 1.4, 2.0 etc).
The major versions such as 2.0 usually include major feature enhancements. This backwards patching allows customers running with an older version to remain on that version while only incorporating bug fix patches.
Same concept as VMware releases 4.0 and 4.1 patches even though vSphere 5 is available.
Regards,
Robert