Hear rate monitor

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I have a 60 Beat heart rate monitor and am trying to use it properly. I had it all attached and connected to the app on my phone. I went for a walk and thought I had it connected to Map My Walk. At the end of 3.5 miles (1 hr 1mins) the hrm app said I burned about 700 calories while Map My Walk said about 500. Now, everything that I read on here says that only an hrm canbe most accurate but I have a hard time believing that I burned that many and it gave a higher reading than the walking app.

Anyone suggestions? Or anyone have experience with a 60 Beat?