Bodymedia assumes that you are burning what you say you are (and you can see and adjust for that based on your armband's readings). Then it can tell you a calorie range based on how much you plan (theoretically) on losing each week (i.e., minus 500 calories per day for each pound planned on losing a week). That burned amount can be a lot or a little depending on your exercise and how active your general lifestyle is (big difference between desk job followed by being a couch potato once you get home versus a manual laborer working overtime).
Sparkpeople assumes you are a sedentary type person and only counts extra burn based on what you claim per week for exercise.
So if you have an active job or life but only count your formal exercise for the extra burn, sparkpeople will still only show a calorie range as if you barely move all day. It would be nice if sparkpeople allowed for selecting a lifestyle multiplier. I assume they don't because people may assume they are more active at their jobs and general life than they really are. Either way, that is probably why you are seeing the difference.
For fun, you can adjust the bodymedia numbers to sparkpeople's calorie range and see what bodymedia's daily burn target is then.
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