The calories burned in MMR is purposely made to be 15-20% lower than reality too make you feel like you didn't ride long enough. That's another motivator.
If you want accurate "calories burned", get a garmin 200 or above. It also has a lot more stuff than just MMR.
I'll second the garmin!
I've got the 310xt and the difference between calories burned using the HRM and when I don't use the HRM is huge. For example, a just under two hour 50 km loop where I *didn't* use the HRM says that I burned 1180 calories. Another ride where I did use the HRM is on the same loop (but minus an out and back): an hour and a half, 38 km, and 420 calories. I highly doubt that my last 10k on that first ride burned 720 calories.
I'm guessing that speed-based calorie calculators don't consider that downhill = fast.