No. In fact, the Laws of Physics don't allow it, Even though you are using water, it is still dependent on a fan and ambient (room) air temperature. And it is impossible for air to cool anything below the air's own temp (without evaporation going on - as in your skin and windchill factors). That would require some form of active air conditioning or refrigeration.
So if your room is 23°C, there is no way the processor (which even at idle is producing some heat) could be 19°C. I would not be too concerned. Those sensors cost about a penny to make. They are very low tech - definitely not precision measuring devices. Are you measuring your ambient temp at your PCs air intake vent?