Earlier this week, a few bits of information leaked out that showed Microsoft would be taking the bold step of making the kernel version go from 6.4 up to 10. This is a big change for the company and for legacy applications, it could impact the software.
Microsoft confirmed the change on an IE support page that talks about changes to the user-agent string, specifically:
Developers preparing for Windows 10 should note that the Windows NT value in the UA string will change from 6.4 to 10.0 with the new release. If you have code that depends on the version number, we advise that you update to allow for the new value.
The user-agent string has traditionally tied back to the kernel value for the OS and when you put the leaked information together with the content from MSDN, it's clear that Microsoft is making the change. Seeing that they are asking developers to update their code too, this clearly shows that this change will be released to the public.
This large jump in version number is likely related to the massive overhaul of the underlying components of the OS to make it the core for all of Microsoft's products. The company is working to consolidate all of its platforms into what the company calls OneCore, which, as the name implies, will be the one core for all of the company's operating systems.
We will be curious to see if this causes any software compatibility issues with legacy applications. As for why did they jump to 10? Well, seeing as their new operating systems is called Windows 10 and there is no difference between calling this version 10 or 7 or 22, it's simply a number.