Don't know why this is not answered yet. It seems not complicated to me.
I'm currently investigation the behaviour of Global Variables myself and ran into this thread.
The GV behaviour as described by Ivan Kharpalev seems normal to me.
The GV contains just a number and does not know if it used for radius or diameter.
When you change the sketch-radius to sketch-diameter, than Solidworks keeps the size of the circle unchanged.
The same thing can happen if you change corner angle from degrees to radians.