A couple of observations, one is the temp sensor isn't all that accurate in some cases. I use an infrared thermometer that I shoot at the output of the heat exchanger. It generally reads about 170F while the temperature gauge reads 182 or so. I have looked at other portions of the block and cooling system and the temps are consistent. The second observation is as rpm increases so does the flow rate of cooling water. So in addition to the thermostat allowing more water through more water is being pumped through the system. Some outboards actually use a spring loaded valve to control cooling at high rpm as the increased water pressure opens the valve allowing more water through.