This adapter is clearly not capable of delivering a null, a power and a ground to the laptop because the connector only has two surfaces. The adapter itself is plastic. So why is it grounded? (Note that this is not specific to this particular brand or model, I see many such adapters.)
I was thinking that maybe on DC side the adapter the ground and the negative is connected but isn't dangerous to connect power to the ground? At another point, say the bathroom you touch a pipe which is connected to the same ground and poof, shock.