This probably sounds stupid, but depending on where that sensor is picking up that reading this could happen when the waterflow is reduced by idling the engine. For instance your gauges pickup could be in a location that is more average in temp and representative of the overall system. Your sensor for the light could be in a volatile or dynamic location where heat changes quickly. The reduced flow from idling the engine could allow temperatures to "spike" in certain locations for a few seconds until the systems re-averages itself out.