The versioning is a bit a long story. We used to use year.month.revision as versioning number. At some point we re-wrote Cura, which was released as 15.06. This was a waay to early release and is widely regarded as being horrible.
In order to prevent confusion somewhat between "Legacy" Cura and "New" Cura, we decided to do a major break and call the new Cura 2.x
The 2.x has been developed further, whereas legacy cura has not been updated in quite some time. So if you want the latest & greatest, download the newest Cura 2.x