Resting energy variation

Hi! It's my first day with my new phone and I'm discovering new (to me) Apple Health features. One of them is Resting and Active energy. I understand Active being the energy spent while moving and resting as somewhat similar to BMR, as in the energy spent for body functions with minimal movements.

I'm trying to understand slight hourly variations in resting energy. Does anyone know the calculations used by Apple and/or why it varies from an hour to the next?

Picture to illustrate.