HRM-is it right?

I have a polar f4 and it usually seems to be very accurate. It even matches with the calories it shows as burned on my zumba game.

I just did 20 mins of trampolining on my big trampoline outside. I looked online to compare and it says I would burn about 70 calories according to my weight (134 lb), but my HRM says 215! That's such a big difference it seems a bit unreliable...

Any ideas why this is?