A good thing to remember is that the temperature rating of the thermostat is the point at which it begins to open. As the coolant temperature rises above the rating, the thermostat will continue to open further in an attempt to hold the coolant at the rated temperature. At the point where it is fully open, it has done all it can do and if the engine generates more heat than the cooling system is taking away, the coolant temperature will continue to rise.
It will eventually settle out when an equilibrium point is reached where heat generated equals heat removed. That point depends on the condition of the cooling system, outside temperature, speed of the air moving over the radiator and the amount of load on the engine and probably a few other lesser things.
I got this from another forum just after I wrote this and it probably says it better. "Using a 160* t-stat will only allow the engine to run at 160* in cold weather- the rest of the cooling system will determine what temperature the engine will run at the rest of the time (as the t-stat will always be open)- which in warm weather won't be anywhere near 160* unless you have a radiator 6 feet wide, 3 feet high and 8" thick (well maybe I'm exaggerating that a bit- but you get the idea). A t-stat only determines the engines MINIMUM operating temperature- NOT its maximum."