Short answer: Heart Rate monitors can over estimate the amount of calories burnt in a day. That being said most online guides (ie. Myfitnesspal, etc) also grossly over estimate the amount of calories burnt.
Longer Answer: Heart rate monitors measure how fast your heart is beating. They use an algorithm to estimate the amount of calories you burn; Height, weight and BMI can help make it more accurate but still over-estimation can range for 12%-33%.
[sub]1[/sub]. Heart rate is a series of factors not limited to exertion. Stress, tempature and caffeine can all influence heart rate and have nothing to do with any sort of physical activity. Weight lifting can spike your heart rate, but doesn't follow the same formula that most monitors use.
They serve a purpose, but aren't the end all be all. A monitor with a chest strap that allows for the input of VO[sub]2[/sub] max will probably give you the most accurate reading. For running I use the following formula that I saw on Runner's World or some place similar....
Weight (in Pounds) x 0.63 x Distance (in Miles) = Calories burnt *******This only applies to actually jogging/running, walking has different co-efficents**********
Example: 235lbs x .063 = 148.05
148.05 x 3 = 444 Calories
YMMV