metering - Why does my camera's automatic mode render both white and black cards as more of a medium grey?

Can anyone tell me why when taking the following 3 photos, in a room with natural light coming in and fill my viewfinder...

a) when I take a photo of a white piece of card it comes up as grey in colour?

b)when I take a photo of a black piece of card it comes up slightly darker in colour? and

c) When I overlap the cards, the white half is lighter and the black half is black?

        R   G   B
White   156 156 156
Black   139 139 139
½ White 176 176 176

½ Black 30  30  30

Thanks