As with the AP/FD system, what you have selected is not necessarily what you actually have. With the reverse thrust levers in the stowed position, the thrust levers will be free to be moved out of the idle position. However, if there is a malfunction, the engine itself may still be in reverse. If I remember rightly, after the Lauda crash modifications were made to Boeing aircraft so that the engine is commanded to idle if the reverser sleeves on the engine are sensed to be out of the stowed position & the aircraft is not on the ground.