Looking through the Windows-specific code, I see lots of instances of *W (like GetVersionW) methods called explicitly, which forces wide characters. Typically I'd expect use use of undecorated names like GetVersion, which lets the compiler configure for or against wide character support (and is future proof!)

See: https://github.com/nodejs/io.js/blob/master/src/node_os.cc#L96

Is there any reason why the code is written this way?

There was a commit from @piscisaureus that describes a change from GetVersionA to GetVersionW -- should calls like this be changed again to just GetVersion?