Dusty hit the nail on the head.
Simple thermostats that rely on bimetallic elements get a lot of criticism but in reality, they work fairly well.
However, because they are a bit slow to react, they maintain more of a range of temperature rather than a point. When you add some other factors, such as the changing amount of mass to control (decreasing amount of lead in the pot) and the addition of large amounts of cooler mass (adding lead to the pot); that range can get bigger.
Simple thermostats work very well when there are limited variables to deal with. For example, a residential tank type water heater does an acceptable job of maintaining water temp in the tank when it's just in stand by. The tank is insulated and there's no influx of cold water. occasionally the water temperature will drop below the set point and the elements will be energized (or the burner activated) until the thermostat opens back up.
When you draw hot water from the tank and cold water rushes in to replace that hot water, the thermostat reacts the same way but there's now a new variable to deal with (the influx of cold water). Because the thermostat is "dumb", it can only react to the temperature it sees. It doesn't know the temperature of the incoming water, the volume of the incoming water or the length of time the cold water will continue to rush in. It only knows if the temperature of the water it is sensing and acts accordingly.
In today's world where it seems like everything is controlled with the assistance of computer, a simple thermostat may look a little outdated. However, in reality, it doesn't need to be very "smart".