while the needle is in the central position all the time, you should also take note that the reading there has been heavily buffered. when the engine is started from cold and as the coolant gradually heats up, at 75degC its already at the central point and it gets up till 89degC and its still at the same point. so its a 14degC buffered reading there. all the time the hot engine runs at 89degC. you can see this in yr test mode.