Microsoft is currently hard at work on an upcoming update for Windows 8.1 which is set to be announced, and maybe even released at the company's annual BUILD developer conference. According to Win8China, who has been behind a number of Windows information leaks in the past, is claiming that Windows 8.1 Update 1 is already being internally tested by Microsoft Employees. http://www.winbeta.org/news/windows-81-spring-2014-update-be-announced-build-april
Even though Microsoft is touting Windows 8.1 as an 'update' for Windows 8, in reality it's a whole new version of Windows, hence the NT Kernel jump. The Windows 8.1 Spring update, also known as Update 1 is going to be more like your traditional update, meaning it's not an entire new OS. According to Win8China, Windows 8.1 Update 1 will not increase the NT Kernel version, meaning it'll sit at NT 6.3, much like Windows 8.1. Of course, this is only a rumor, so take it with salt. http://www.winbeta.org/news/windows-81-spring-2014-update-will-not-increase-nt-kernel-version