oil filled heater question(electricians)

i have an oil filled electric heater in a cold room of the house. heater has 3 power settings and a thermostat. i put a magnetic thermostat on it to see what temp it ran at. i had contradictory results. when the heater is set on medium it will run about 130*avg. when it is set on low it runs at 175*avg. the heaters thermostat stayed set at 3/4 for both tests. i was wondering why this is. also wanted to ask the electricians if these heaters were more efficient at high or low power settings.

Ron, I can not explain the thermometer readings with out knowing where the thermostat is located.
To answer your question about efficiency; It should not matter much. At the high setting there will be a greater line loss between the heater and your meter but it is not likely to be significant.
If your utility company has time of use billing then you might consider using the high setting at night when it is coldest and rates are lower. This may not apply to your heater if the room temperature is low because of the thermostat location sensing the heat of your heater and not the ambient air temperature of your room.
I wish I could give you a better answer.