The container might be different than the liquid if the air temperature is different than the liquid. If it is a hot day, and you are measuring a cold liquid, the wall of the container might be conducting heat into the system. Similarly, if you are heating a solution on a hot plate and the thermometer is touching the container, the wall of the container will be hotter than the liquid in most cases. To properly measure a liquid, you should have a mixing bar or some other form of agitation and suspend the thermometer close to the center of the liquid.