Heaters are measured in BTUs, not degrees. British Thermal Units.

1 BTU is the amount of heat energy necessary to raise one pound of water one degree Farenheit. I have a LOT more heat energy in my pool at 82 degrees than in a burning match at 750-800 degrees.

So if last year the water was 10 degrees warmer but the flow this year is 10 times faster at 2 degrees warmer, you are getting double the heat energy.