Heart monitor doesn't seem accurate!!!

I was wondering if anyone can help me with my heart monitor. I have a Polar FT40 and I wore it for my spin class the other night and it said I only burned 190 calories (no way) . It was 50 straight minutes and I was doing everything instructor was. She said cant be right hers said she burned 580 . I know everybodys different but come on I burn more than that on the treadmill. Any suggestions what could be wrong?