Heart rate monitor incorrect?

Hey all, so I have the polar ft4 heart rate monitor. I read the instructions and wear it to the gym. I'm wondering how accurate it is though. I am decently fit, I'm not too overweight, when I am on the elliptical for 30 minutes the heart rate monitors on the machine tell me I burn about 300 calories, but my polar monitor says like 450-500 and after lifting within a hour I am at 800 calories. I feel like this is far more than I am burning. I've adjusted the tightness and everything but it usually is a high number. Could it be that I am burning that much or is my polar heart rate monitor not Accurate?