Why is it COR instead of COM3?

This is mildly amusing. The internal name for what MS now calls COR (Component Object Runtime) was once COM3. They changed that before reasing. Why? Don Box explains:

On at least one version of Windows, if you created a directory called COM3 you could never delete it (COM3 is a reserved name for a serial port).

That's a legacy installed base issue raising its head for you.