I could see how it is double walled, but I wasn't sure how the coolant flowed through there. Is the intent of the design to control how the coolant flows through the chamber, forcing the coolant either from the sensing holes (bypass closed) or the through the bypass valve to flow over the end of the thermostat before it heads off to the pump? I guess if the top was just an open space with two ports on it the coolant could just short-circuit and flow across the top of the chamber without getting to the actual thermostat.

The thermostat is cycling between open and closed until it reaches more of a steady state condition with a combination of cold coolant from the radiator outlet and hot coolant from the sensor holes controling its position. That blending of coolant flows means that the coolant temperature in the engine will always be higher than the thermostat's operating temperature.