Who's good with HRM's?

I recently got the Polar F4 and I have a question... Today I wore it at the gym while I ran for 30 minutes. I burned 313 calories and thought "I'll leave it on while I go grocery shopping quick just to see what happens." My heart rate stayed slightly above 100 during my walk/jog through Walmart and I ended up burning over 100 more calories by the time I got home. My question is... Are the calories calculated by my HRM over and above my BMR? As in, does the HRM subtract the calories I would have burnt if I was just sitting around, or is the number on my HRM my BMR+any added activity? Thanks in advance for any input!