Why is active calories different than total calories

Your active calories are the ones you earn as you are active and exercise/work out.

Your total calories are active and resting added together.

Track daily activity with Apple Watch – Apple Support

Remember: Apple Watch calculates both active and total calories based on your activity level, age, height, weight, gender, and heart rate. You can edit that in the Health App.

Get the most accurate measurements using your Apple Watch - Apple Support

Well you see, because Total Calories include both the calories you're burning from the activity and the baseline calories you'd be burning anyway, Active Calories increase more slowly because they're only counting the additional calories burned from exercise.

Your body always burns calories, even when you're resting. When you start exercising, total calories add up faster because they include both the calories you burn from exercising and the ones your body uses just to keep you going. That's why total calories increase more quickly than active calories. Hope this helps your understanding!