Hello Panda_destroyer,
Thank you for using the Apple Support Communities! I see you have a question about the Total Calories count you're seeing in the Activity app on your iPhone. I'll be glad to see if I can provide you with some information about this.
You are correct; what you're seeing is the total calories burned for that day. As far as how it's calculated, there are a number of factors. This might answer your question: Get the most accurate measurements using your Apple Watch
"Your Apple Watch uses your personal information — such as your height, weight, gender, and age — to calculate how many calories you burn and more.
To update your personal information, open the Apple Watch app on your iPhone. Tap the My Watch tab, then tap Health > Health Profile. Tap Edit, then tap the item that you want to change."
I hope I've answered your question!
Cheers.