At the launch of Windows 10, many people were wondering why Microsoft decided to skip calling the OS 'Windows 9', which would have been the obvious choice going by past naming conventions. Some people claimed the updated OS is named this way because it's such a significant upgrade over Windows 8.1, but that might not be telling the full story.
According to a self-proclaimed Microsoft developer on Reddit, during testing of the next version of Windows, the company discovered a number of third-party programs that were lazy with their version checking. Some apps were simply checking if the name of the OS reported by Windows started with "Windows 9", and configuring themselves as if the OS was either Windows 95 or Windows 98.
This caused compatibility problems when the new version of Windows was called 'Windows 9', as these apps believed they were running on a horribly outdated OS. This method of checking the version of Windows isn't one Microsoft advises developers use, but the problems during testing may have swayed the company to name their new OS 'Windows 10' regardless.
Microsoft has previously run into compatibility issues arising from in-app version checks, which is why all modern versions of Windows report themselves as Windows 6.x, despite being named with a higher number. Windows 8.1, for example, is actually labeled as Windows version 6.3 by the OS. Running into a similar issue with Windows 9 isn't unbelievable, though choosing to rename an OS because of it does seem somewhat far fetched.
That said, some Redditors did point out that the lazy version checks are found throughout a large number of applications simply by performing an online check of publicly available code. This does make this theory behind the name of Windows 10 more credible, although Microsoft isn't likely to confirm it any time soon.