Yes, a HRM is a wake-up call for a lot of people. I see a lot of people really overestimating their APs, it's their problem though, not mine.
So to the original poster, your calories burned at the HR you mentioned, actually seem fairly right to me.
With time and consistency in workouts, you will really get a feel for things. For example, last week I did the same cardio workout each day (I'm experimenting.) I also worked out at the same intensity. On the ArcTrainer (elliptical) I did ~ 45 min, at an average HR of 77%, and burned 400 calories (that was almost exactly the same for M-F last week.)
Yesterday I did the same workout (same music CD) for 45 min, but I worked at a lower intensity, only 70% MHR, and I only burned 311 calories. So dropping my intensity changed how many calories I burned in the exact same amount of time.
Today I did 65 min, at 73% MHRM, and burned 500 calories.
The more I track, the more consistency I see in what I burn.
BTW, I use a Polar f11 and I track on the polarpersonaltrainer.com website which I can upload my data to.