I did not know that the user agent for Chrome reported Windows 8 as "Windows NT 6.3"
That has nothing to do with the used browser at all. All operating systems have all their own, unique ID. If you would like to learn more about user agent strings from operating systems, you might like to visit: http://user-agent-string.info/list-of-ua/os
Windows 7 is the version "Windows NT 6.1" for example, but if the forum declares all "Windows NT 6.1" - Versions as "Windows 7", it is based on the fact, that the forum is modified with the information, that "Windows NT 6.1" is the name of the os "Windows 7". ^^