I researched a little more and found that before Vista the winver command used the CurrentBuildNumber key instead of CurrentBuild key. In fact before Vista CurrentBuild was marked as obsolete by Microsoft. But that changed in Vista, when Microsoft started using CurrentBuild key instead of the CurrentBuildNumber key. Though still today CurrentBuildNumber key actually gives the exact version (for backward compatibilities maybe?). Therefore why not use CurrentBuildNumber? At least CurrentBuildNumber gives the right info. The only problem is that we have to only use that key to determine the windows version (Eg - Windows 10, 8.1, 8, 7, Vista, etc.) and to do that we have to use a very long switch or if-else statement. 

https://en.wikipedia.org/wiki/Comparison_of_Microsoft_Windows_versions#Windows_NT has the whole list with the column "RTM build" as the CurrentBuildNumber value.