FBA fee is never a flat fee

I’m curious to learn more about FBA fees. I’ve been selling on Amazon using FBA for over 2 years but my FBA fee fluctuates everyday. Even when I look at the monthly average FBA fee it is higher than the stated flat fee that Amazon says it is charging.

Same goes for my referral fee. It should be 15% of my sale price given the category I am in but it also changes everyday and the monthly average is more than 15%.

Does this happen to anyone else? Or does anyone know why this happens?

What am I missing? Appreciate any help.