30% calories difference in same workout?

Francl27

I go to BodyJam once a week. The last 3 weeks I burned between 480 and 520 calories in one hour. Today it was only 340.

I don't understand how it's even possible. I've lost 2lbs but that's like 1% of my weight (and I adjusted the HRM). It stopped working at some point (told me to check the strap or whatever), but I don't think it was more than a couple minutes because I check it regularly. It's a polar P4. I know I need to wash the strap too, which I'm doing now, but still.

Any idea?