The difference is based on Microsoft's life cycle start date and its decision to support platforms for 18 months. Microsoft pegs the Windows Phone 8 start date at December 14 and Windows Phone 7.8 at February 9.
Windows Phone 8 is the latest and most featured-packed version of Windows Phone. Windows Phone 7.8 was the option Microsoft offered to former Windows Phone 7 owners who didn't want to buy a new device. There's no clear upgrade path to Windows Phone 8 from Windows Phone 7, so Windows Phone 7.8 provides some, but not all, of the features in Microsoft's operating system.
That Microsoft is ending support for its latest operating systems next year isn't all that surprising. The mobile space is driven by rapid operating system updates. Apple, for example, launches a new version of iOS each year. And since most consumers update their devices every two years, needing to keep operating systems updated for several years doesn't make much sense.