Calories burned in exercise decreasing?

Okay, so this may be a silly question but I want to check if my thoughts are correct. I have been logging on mfp, my food and exercise for almost 2 months and I have lost 14 lbs, which I am happy with. I have been doing Insanity and I am on my last week! I have been using a heart rate monitor to get my calories burned, which since Insanity is crazy, the calories burned have been good. When I started month 2 of Insanity, I would easily burn in the high 500's for a 60 minute workout. Now, I only burn in the 400's. Is this because it is now easier for me because I am more in shape? See silly question, but I just want to make sure that is right and that I am not slacking off on the exercise. I really do feel that I am still putting my all into it.

Thank you
Heather