Calorie burn confusion

linnypp

I've had a Microsoft band for the last few weeks - love it for keeping track of runs etc but, I'm confused about the calorie burn it's recording.
In a day (not including exercise) according to the band I'm burning around 1600 - 1700 calories - yet trying to maintain at 1500 calories (with pretty accurate calorie recording) I'm putting weight on.
A few days ago I walked a fast 3 miles (50 minutes) and it said I'd burned 250 calories. Yesterday I did the same walk very slowly (over 2 hours) with lots of stopping (children with me!) and the band said I'd burned 750 calories. Does that seem right that the burn was so much higher for the same distance? I'm not sure now if the band is recording calories accurately, and would love to hear what others think. Anyone else out there using the band and have any thoughts about it's accuracy?