What a sedentary lifestyle does to your calories
Your body burns energy in four ways: keeping you alive, digesting food, planned exercise, and everything else you do while moving about. That last part is called non-exercise activity thermogenesis, or NEAT: walking to the station, taking the stairs, standing, fidgeting. It varies more from person to person, and from day to day, than any of the others, and a desk job pushes it down.
That is why two people eating the same food can have different results, and why a day of meetings can leave you hungry but having burned very little. A block handles it in two ways: eating targets set for your actual desk days, and a step target that raises your NEAT without asking for another gym session. The glossary entry on NEAT has the short version.