Calories burned

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Just curious...I typically do a 45 minute workout a day, they very. But when I do my workout DVD (jullian Micheals, Teabo, etc.) they are about 40 minutes and I typically burn about 400 calories (according to HRM polar f6)

My question is why would every once in a while I burn 525. I don't think I am working any harder. Is my body tired and working more? Does anyone one else have that happen?