Calories burned and heart rate

Echosapa

Hello all. I just recently started working out playing Zumba 2 on the Wii for the last two weeks. I have been monitoring my heart rate with a heartrate monitor and I have noticed something. I was burning about 700-900 calories an hour, now I'm burning about 400-600 calories an hour ( I have my hrm set low for weight but I'm giving the numbers that this says). I thought for a a few workouts maybe I wasn't giving it my all, but today I noticed that my starting heart rate was about 20 lower than it usually is. I know when you are a conditioned athlete you have a lower starting rate then most people, could my heart rate being going down that fast? Or could it be that since i have done the same workout those weeks that I'm not getting my heart rate up as high to burn those calories (I hear this happens because your body becomes "bored") and the starting rate is just a coincidence. Seems to early for that much of a change. Wondered if anyone knew.