The thermal well is submerged with water in all directions except where it contacts the kettle but usually this is far enough away from the sensor that you can consider the thermal well to be completely surrounded by water. Therefore the tip of the well where the sensor is will be very close if not the same temp as the water. Once in equilibrium there will be no thermal transferred through the thermal well or temp sensor, whereas there will always be heat transferred through the kettle wall.
At my last company I worked on a problem where we were trying to roughly measure the temperature of water or steam inside of a pipe by measuring the outside surface of the pipe. We were able to come up with a solution that worked half ass when there was no airflow, but as soon as you added the slightest airflow the delta between inside pipe temps and surface temps changed by a lot. The surface temp of kettle will vary depending on air temperature and wind speed.