Whenever anyone says "half an hour of X burns Y calories" what they usually mean is that "you burn Y calories during half an hour of doing X" which is actually different. The calories that you burn aren't all excess calories. If you burn 170 calories doing yoga and you would've burned 70 calories just sitting on your ass then you've only burned 100 calories. But they'll "reward" themselves with a 170 calorie snack regardless.