The Fitbit Community is a gathering place for real people who wish to exchange ideas, solutions, tips, techniques, and insight about the Fitbit products and services they love. By joining our Community, you agree to uphold these guidelines, so please take a moment to look them over.
It's because the calories are based on your HR. If you change intensity or Sense has problems with reading your HR or there's any other factor causing the HR will be different, then your calories will differ, too.