<human>: Condense the text into a few key points

Hi guys, I have a question regarding my BMR.  Last year I purchased a Bodybugg, a device that is strapped onto the upper arm to detect how many calories the user burns.  I used it for a while, then stopped for a few months.  Recently I picked it up and started using it again.  To my surprise, I burn a lot more calories a day than I expected.  Apparently, on a day when I do nothing but stay at home, lie in bed, and sleep, with minimal walking within the apartment, I burn ~1,800 calories...I am doing almost nothing physically, so I am guessing that's my BMR.  And on a regular school day when I walk to school, I burn ~2,300 calories.  This seems very unusual to me because last year when I first started using the Bodybugg, I was burning a lot less-- ~1,200 calories on a lazy day (BMR), ~1,600 on a school day.  At that point in time, I was the same exact height, but 5 lbs heavier (95 lbs).  How come I burn so many more calories than expected now, even when I'm doing nothing at all?  I don't think the Bodybugg is broken-- I recently lent it out to my boyfriend for a week, and his results seemed pretty accurate (5'7", 120 lbs, burned ~2,400 cal/day).
<bot>: On a day I do nothing, I burn 1,800 calories instead of the expected 1,200.  Why?