Why class variables are shared by subclasses?

	class A
	   @@cv = 3
	   def A.ppp
	      p @@cv
	   end
	end

	A.ppp

	class B < A
	   def B.t
	      @@cv = 5
	   end
	end

	A.ppp
	B.t
	A.ppp

	class C < A
	   @@cv = 7
	end

	A.ppp
	B.ppp
	C.ppp

Produces 
	3
	3
	5
	7	
	7
	7