Different calorie numbers

I've been using a HRM (the wrist watch kind without a chest strap) and the MapMyWalk app while taking my daily walk. The HRM gives me a calories burned number about 150-200 more then the app and even MFP.

I know the heart rate monitor is supposed to be the most accurate way to get this number, but since I have two other sources telling me differently I'm starting to second guess my monitor. I usually get a reading on the monitor every 5 minutes or so during my walk.

Any suggestions as to what number I should go with? I'd rather not buy a new HRM if it's not necessary. This one is only about 6 months old!