Several Weeks of Maintaining and a Heart Rate Monitor Distraction

I'm currently having flash backs of my 10 week struggle to get beyond the 20-something pounds lost mark. I've been hovering in the 40-something pounds lost for several weeks now, and quite frankly, I'm getting annoyed. I really don't think I'm impatient about losing, and I work really, really hard every day to work this chubby rear off, but something in my body just doesn't want to cooperate at all. I've tested various theories - not eating enough, eating too much, not eating the "right" foods, etc - but nothing seems to help facilitate the process. Because of this frustration (or perhaps more as a distraction), I decided to try again with a heart rate monitor. I'd been pondering picking one up for a bit, and we happened to be at an REI garage sale where I was able to get one for under $30. This morning, I tested it out at the gym.

Goodness knows I haven't been to the gym in quite awhile because I've been amusing myself with kickboxing and cycling so frequently, but I figured it was a good place to test it out. By the time I'd finished my 45 minute sweat-fest on the ARC machine (it's basically a combination of an elliptical and stair climber), I was drenched head-to-toe, and according to the monitor, had burned 477 calories. Really? My online logger claims that I "should" have burned 871 calories, while the machine itself at the gym stated that it was 794 calories burned. Can it really be that off, and what is more accurate?

Here's what I've read online:

"Calorie burning isn't determined by heart rate, it's determined by the number of muscle cells that are activated to perform a given activity. It's the working cells that actually use the energy (calories) and consume oxygen. When working muscle cells need more energy and oxygen, your heart rate goes up to deliver these things to the cells via the blood stream"

So, what is this telling me exactly? I understand that the number of muscle cells used to perform an activity are important, and this is a machine that uses major muscles, but yet the calories burned seems off. How do I know what to believe though, and how can these numbers be SO different? I don't mind seeing that the number burned is lower, but I also want to know that it's accurate - otherwise, there's no real point to wearing the darn HRM. I've also looked into "power meters" as many claim that it's the only way to get a more accurate reading of calorie expenditure.

Although I'm not entirely sure where this is taking me, or if it will be of benefit over the long haul, it's been a fun little distraction to see what I'm capable of doing, and the rates at which I expend energy. Now, if I can just get out of this rut with weight loss, I'll be a happy little camper.