Well, if you know the history of NT Operating systems, then NT 4.0 was being developed into a new operating system. It was going to be called NT 5, but by the time it was out on stores, they called it Windows 2000. This was also the predecessor of XP, and since it follows the file-system and memory structure of NT/2000, it is detected as NT, version 5.1 build 2600, if you have Service Pack 1.
A little combersome, but still an explanation
Josh Dinkel