HRM overestmating?

There are two calculators on that site though. I assume to use the first one since it's a lower number.

I've noticed that people say MFP overestimates calorie burn, but I've found that the estimates I get from MFP are usually lower tan what my HRM says I burned.

MFP is about the least accurate method there is. It can't possibly know all kinds of factors that go into an aerobic calorie burn. MFP is not about being the most accurate. It's about adopting a lifestyle that yields results over the long term. For example, if you are hitting the elliptical for 30 minutes, MFP will always give you the same number. Doesn't matter whether you are doing 50 strides a minute or 200. Doesn't matter if the resistance is on 1 or 20. Doesn't matter if the elevation is flat or highly inclined. MFP will only be accurate by accident. If accuracy is your principal concern, only use MFPs numbers as a last resort. For the 50 strides a minute crowd, MFP likely over estimates. For the 200 strides a minute group, MFP under estimates.

On Wednesday, I did approximately 25 minutes of a Walk Away the Pounds workout, which I logged as "low impact aerobics." My HRM said I burned 234 calories. MFP estimated 175. (Granted, about 6 minutes or so of the workout WAS jogging, the numbers still don't match up.)

So you told MFP you did an exercise, which you didn't actually do, and it gave you a different number than your HRM? And if you were just simply walking, you probably spent most of the time below the aerobic threshold, so your HRM won't be accurate anyway.

I'm just concerned about this. Is it likely that my HRM is overestimating my calorie burn, or could MFP actually be underestimating for me.

Using a HRM is more accurate, but if you don't know your VO2 Max, and using that to calculate it, it will be off as well. Plus, a HRM is only accurate in the aerobic zone. Once your HR climbs above 65% Max, it begins to skew as your body begins utilizing more and more glycogen as fuel. And, if you have anything that is adjusting your HR (caffeine, workout supplements, thermogenic boosters, diet pills, etc), it can be off as well. Remember that a HRM only measures your heart rate. It doesn't measure caloric consumption. That is a calculation based on your heart rate, while it is in the aerobic zone. Too high or too low, or if you're doing anaerobic exercises (such as weight lifting), and it is no longer accurate.

So, basically, my Miffin BMR at 22 years old is 1605 calories. My Harris BMR if I put my age at 33 is 1606 (34 is 1602), so it will be more accurate if I tell Polar that I'm 33?

Both of these calculate your BMR and don't really have a significant effect on the calories burned during exercise. Fiddling with the age to get a number that you want to see is just a math problem, and isn't going to give you more accurate info.

Obviously, using BF% will make it even more accurate, but right now, I'm more concerned about my tools (Fitbit/HRM/MFP) matching up as closely as possible.

You will drive yourself crazy trying to get all of these tools to match up the same. They all use different formulas, and different methodologies to get their numbers. And most of them don't tell you what they use. And their numbers can change based on your effort.

Can a fitbit strapped to your arm possibly give you an accurate measurement on a bike ride? Can it tell the exertion level required to pedal 80 strokes a minute uphill vs flat land vs down hill? Can it tell the difference if you're jogging uphill?

Save your sanity. Pick a method and stick with it.

Bottom line, I bet you'll find most of these only vary by about 100 calories on any given day. If 100 calories is going to make the difference between whether or not you're losing weight, you're doing it wrong