HRM H7 app calorie burn problem

jimboodee2

Hi I have the polar H7 HRM which I have been using with the polar beat app - in my spin classes I have been burning approx 400 kcal a session I am overweight so expected it to be slightly higher. So I was doing an insanity class this afternoon and downloaded mapmyfitness app to compare the two. Myfitness says I burned around 840 kcal and polar beat says 370 kcal. This is obviously a big difference and don't know which one is accurate any advice on these two apps is appreciated :-)