Did you try measuring the power at the wall end of the cord while the heater was running? Most space heaters aren't very smart, and it wouldn't surprise me if the draw varied wildly depending on the power supply and how warmed up the heater is (I know mine will go between 1 and 1.5kW plugged straight into the wall as it warms up).

I'd be surprised if a 100-foot extension always consumed 850 watts.