how does body media determine "target" calories?

It came to that number based on the information you entered--your personal stats like age, height and weight and your goals, how much weight you want to lose each week and how much you want to eat each day.

2900 calories a day is a lot. If you have an active job (mailman, for example) or are heavier to start with (heavier people burn more calories) or workout a whole heckuva lot than you can pull this off!

If you said you wanted to lose more than a pound a week, say, 2 pounds per week, you might want to go down to one pound per week.

At 1500 calories a day, I'd have to burn 2500 calories a day to lose 2 pounds in a week. I workout every day with moderate to vigorous intensity and a 2500 calorie a day burn would be a real challenge for me. I also have a desk job.

There's a body media group, if you want to join!

I'll give you an example of the "early morning high burn" scenario I mentioned up above:

This morning I went for a 4mile run, did one hour of strength training, spent an hour or so cooking and cleaning the kitchen, then went for a 4 mile walk. I synced and it showed I had an extra 1675 calories.

Since then I've been sitting down doing photo editing for about 5 hours. It has dropped my extra calories to 1353.

So if dropped the numbers after you sat down? I thought it gauges (somehow) what you are doing rather than estimating. Isn't that what the armband deal is for? The sensors on it. What do they do then if not gauge your movement.? ETC

It does keep measuring what you are doing.

But MFP is projecting the day based on what you've hit so far.

So if he hit all that calorie burn in first 8 hrs, MFP is going to think the day is that burn x 3 for 24 hrs.

But then at 12 hrs, he's added hardly any more additional calories, so MFP is estimating day with that burn x 2.

You are misunderstanding that the adjusted calories seen on MFP are NOT just the exercise, it's everything above the non-exercise maintenance MFP thought you'd hit.
As the day goes on that above math is done - Body Media still tallying up your calorie burn based on sensors.

Nothing regarding goals comes over to MFP - only actual burn values.

In general, I stick with my BM calorie allotment. I just log my food into MFP. I never eat back all my exercise calories, because the BM calories imported into MFP are also total calories burned just by existing, which the calorie total already accounts for.