If you’re a regular user and not a developer, you may not have noticed that internal Windows version identifiers have been stuck at 6.x since Vista.
Vista was a disaster, with one of the biggest problems being software compatibility. Programs that worked fine on Windows XP no longer worked on Vista. One reason that happened was that Microsoft bumped the internal version of Windows from 5 (XP) to 6 for Vista. This caused a lot of software to get confused and stop working.
This made Microsoft much more reluctant to make major changes to the internal version number for subsequent releases of Windows. The internal version identifier for Windows 7 is 6.1. For Windows 8, it’s 6.2, and for Windows 8.1, it’s 6.3.
Microsoft also got to work on finding better ways to get around software compatibility issues, and developed the Application Compatibility tools, which include a simple method for tricking software into thinking it’s running on a different version of Windows.
With Windows 10, Microsoft apparently plans to get back to internal version numbers that make sense, and they’re also using this opportunity to finally make the internal version match the external version. Windows 10 will be identified internally as version 10.