There are 2 different methods to calculate your calorie range: one is based on your calories burned goal, and the other is based on the exercise you're tracking. So if your calories burned fluctuates a lot from day to day, you might consider changing to that method. To do this, click on Account Preferences at the top left of your Start page, scroll down and you can change it from there.
Hope that helps,
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford
"No matter how slow you go, you are still lapping everybody on the couch."