HRM not adding up?

I bought a HRM recently. I am not sure if this is normal or not, But I just did a 10 min workout with my 2lb jump rope. Checked my heart rate during a couple marching intervals, and it was at 88% (HI) but it says I only burned 64 cals. Whereas when I check online with calculators it says it should have been over 100 cals that I burned. So do I trust the HRM? Or could it be off?