It's because you've spent LESS time in your lifestyle... there are only 24 hours in a day. So when you don't exercise at all for 24 hours a day, you burn, let's say 2400 calories just in your lifestyle (I'm doing this for easy math.)
Then you exercise for an hour and burn 300 calories. BUT, that's now an hour you HAVEN'T spent burning lifestyle calories. So fitday takes 100 calories away from the 2400 (23 hours lifestyle calories now = 2300) and adds the 300 calories you spent exercising. So, technically, that hour workout only ADDED 200 calories.
Hope this helps.