Apple watch activity

Hi, i was thinking about how to read my apple watch. I thought the appel watch calculates your average kcal per day and when you do some activities, your average will go up. For example, when i do nothing the whole day my kcal will be 1500. So when i do an activity it will go up. But then my kcal per day are really high when i work or go to the gym. But now i just put on my apple watch for 1 minute and it says that i have burned 875 kcal ?!!! i dont understand anymore how to calculate. I alteady checked my settings en kalibrated the watch and iphone.