Why are my total calories vastly different on two similar activity days?

Why do my total calories change so much - for example by 500cals on two similar activity days?

I can have slightly more move calories and steps one day but total calories 500 less than the previous day with less move cals and steps?!?

I didn’t change any weight details etc that would alter calories burned in that way.

it’s very annoying and difficult to eat calories accordingly when the difference is so big and I expect a similar calorie burn as the day before as move / steps etc are on track to be more