Why is Environment.OSVersion hardcoded to 5.1.2600.0?

Guys,

I've been trying to understand why the System.Environment.OSVersion
property is hard coded to 5.1.2600.0 but need someone less naive than
myself to show me the light.

I know it used to be 0.0.0.0 and that it now matches Mono's
implementation, but I don't fully understand why it has to be hard
coded.  The best I've been able to come up with is that because some
OSes like GNU/Linux don't have strict OS version numbers (where I guess
the closest thing to an OS version number if the kernel build), although
since Windows and MacOS do have strict version numbers I'm guessing the
value could be grabbed from an internal call at some point.  

Any clues would be much appreciated.

Cheers,

Mark