It's the same reason that my 15 amp table saw on a 10 amp breaker only pops the circuit about once or twice a session. The figures are maxes. My tablesaw only pulls 15 amps when the dummy operator overloads it. Same thing with the refrigerator compressor. Yes, I know that I have to run another circuit soon. And hopefully you trun the heater off when you leave the house so you don't come home to melted ice cube trays or much worse.