I just got a Polar A5 heart rate monitor, and I tried it out on the treadmill. The Polar said I burned 440 calories, the treadmill said 270 and caloriesperhour.com says 330+ calories. I'm curious: is the HRM any more accurate than the other methods?

(Stats: did 30 minutes at 12% incline at 3.2 mph, mostly at 80-85% heartrate. I'm 6'4, 198lbs. and 34 yrs old. Treadmill doesn't calculate age/weight/height.)