The coolant temperature sensor measures the temperature of the coolant as it passes through the coolant manifold. If there is no coolant it is merely measuring the temperature of the manifold and the air within it.
Say there is a blockage, then the temperature of the coolant will increase and radiate through to the sensor, if the engine is working hard and getting hot it will do the same thing, same with a thermo fan failing, same deal.
With a leak, the remaining water in the system may not get hot until way past the point in which the coolant sensor can't really get effected by the coolant any more..