How does Apple Watch calculate total calories burned accurately?

Hi! I am looking at the calories listed on the Heath App tied to my Apple Watch and I feel like they are low. Please help me understand. I had an active day yesterday and far exceeded my Move goal. Move shows 856 calories. I thought Move is added to BMR to get total calories. I believe Apple uses the Harris Benedict model to calculate BMR. Using a Harris Benedict calculator, my BMR calories should be 1179. 1179+856=2,035. So why is Apple saying I only burned 1705 calories? This is making me sad! Thank you for your help!

[Re-Titled by Moderator]