I found out the same thing when I did my AT testing - my body is efficient at working so that means I burn less calories working out than my Polar HR monitor was telling me. It's great if I want to go long distances, but not so good for losing weight.

I also found out that my Polar's Vo2max test was about 10 points off! I did my test on the spin bike, and didn't learn until later that my LT for running is different than my LT for biking. Not sure if I want to pony up the cash for another LT test to find out my run LT yet - they say you can just add 10 beats to your bike LT and come pretty close. But I also found it to be very valuable information.