HRM reading/calorie burn confusion.

normalme

I bought a heart rate monitor for doing Insanity. I've also used it on exercise machines. When I input my details etc into a machine I pretty much do 90-100 cals every 10 mins according to the machine (without HRM). My HRM monitor is saying I'm burning about half that. When I do insanity it's said my MHR has been 254! As a 25 year old female surely 100% of my MHR is 201 (226 - age)? therefore I should... be dead. During stretches it usually gets back down to late 90s early 100 bpm before the crazy insanity stuff starts again.
It says my resting heart rate is around 68 and when I run or cycle it seems like it's giving a pretty realistic reading, is it possible my HRM just can't give decent readings during insanity workouts? I've seen no bad reviews on the hrm, it only goes to crazy numbers during insanity. But still the calories burnt calculator on both my HRM and online average heart rate, calorie burn calculators say I only burn around 300 an hour!
I don't get it, I can't work any harder, 300 cals an hour for intense workouts seems tiny compared to what the machines or the calories burn estimates on here say. It's not like I'm really light, so burn nothing either. I weigh 117 and I'm 5ft2. What is the most accurate way to find out what I actually burn? 300 cals an hour is pathetic, especially when you hear people quoting 700-1000 an hour during insanity workouts. It's not like I'm being lazy, I'm on week 4 and pretty much keep up with the people on the video now (except for level 2 drills and power jumps= pure evil muscle failure) I would ask if a rapid recovery time can bring down the average bpm during stretching and breaks and thus give a lower average reading and calorie burn, but I don't think my recovery time is particularly rapid. Do these numbers seem right? I've seen people who only weight 100lbs say they burn more, but I can't exactly increase my heart rate to burn more if I'm really reaching 254! When I run I max out at about 200 bpm and average 155-165, this seems normal, so why so little at the end of insanity?
HELP!
Charlie